stihl carburetor manual
TRANSCRIPT
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1. Safety Precautions 2
2. Introduction 2
3. Assignment 3
3.1 Designation 8
4. Purpose ofCarburetor 8
5. Construction/Function 9
5.1 Fuel Pump 95.2 Operating Conditions 11
5.2.1 Full Load 115.2.2 Idle 125.2.3 Carburetor with
Fixed Idle Jetand Primary MixtureControl 14
5.2.4 Carburetor withBalanced Idle System(Fixed Idle Jet andAir Bypass System) 15
5.2.5 Part Load 165.3 Problematic
Operating Conditions 175.3.1 Starting 175.3.2 Temperatures/
Humidity 195.3.3 Acceleration 195.3.4 Dirty Air Filter/
Compensator 205.4 Special Features 21
6. TroubleshootingChart 23
7. Servicing 25
7.1 Leakage Test 257.2 Cleaning 257.3 Pump Diaphragm 267.4 Metering Diaphragm 267.5 Inlet Needle 27
7.6 Main Jet 287.7 Full-Load Fixed Jet/
Part-Load Fixed Jet 297.8 Control Valve 297.9 Manual Fuel Pump 307.10 Check Valve 307.11 Carburetor Body 317.12 Throttle Shaft 327.13 Choke Shaft 347.14 Accelerator Pump 357.15 Lever Mechanism
on Carburetors withAutomatic Choke 36
7.15.1 Throttle Shaft/Choke Shaft 38
7.16 HL-324A / HL-327D /LA-S8A Carburetors 38
7.16.1 Throttle Shaft 437.16.2 Choke Shaft 447.17 WYL-63 / WYL-73
Rotary ValveCarburetors 45
7.18 Standard Setting onWalbro Carburetors 50
7.18.1 WT-283B, WT-323,WT-325A, WT-326C,
WT-360, WT-396,WT-403A, WT-493,WT-498A, WT-503Fixed Jet Carburetors 50
7.18.2 HD-13A, HD-16, HD-17,HD-18A, HD-21A, HD-22,
WY-69, WT-329, WT-330,WT-367, WT-393,WT-489, WT-492,WJ-69 Carburetorswith Limiter Caps 51
7.19 Standard Setting onZama Carburetors 54
7.19.1 C1Q-S30C, C1Q-S37A,C1Q-S41, C1Q-S42,C1Q-S34B, C1Q-S36BFixed Jet Carburetors 54
7.19.2 C1Q-SK7, C3A-S39A
Carburetors withLimiter Caps 55
7.20 Standard Setting onTillotson Carburetors 59
7.20.1 Fixed Jet Carburetors:HS-275A, HS-279B 59
7.20.2 HL-366A, HS-280A,HS-281A, HT-12ACarburetors withLimiter Caps 60
7.21 Flow Chart 64
8. Special ServicingTools and Aids 65
8.1 Special Servicing Tools 658.2 Servicing Aids 65
1999 Andreas Stihl AG & Co., Waiblingen
CarburetorsContents
Carburetors 1
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If the chainsaw or power toolis started up in the course ofrepairs or maintenance work,observe all local and country-specific safety regulations aswell as the safety precautionsand warnings in the owners
manual.
Gasoline is an extremely flamm-able fuel and can be explosivein certain conditions.
Improper handling may result inburns or other serious injuries.
Warning! Do not smoke or bringany fire or flame near the fuel. Allwork with fuel must be performedoutdoors only. Spilled fuel must be
wiped away immediately.
Warning! To avoid the risk ofaccidents and personl injury, takeextreme care when performingadjustments without the cuttingtool guard or deflector.
This service manual contains des-criptions of repair and servicingprocedures as well as functionaldiagrams of most carburetorsused in STIHL gasoline powertools.
While carrying out repair workyou should make use of the latestillustrated parts list of the powertool concerned. It shows the instal-led positions and the assembly se-quence of individual componentsand assemblies.Parts lists on microfilm andCD-ROM are always more upto date than printed lists.
A fault on the carburetor mayhave several causes. Consult the
troubleshooting chart - see 6.
Refer to the "Technical Informa-tion" bulletins for engineeringchanges which have been intro-duced since publication of thisservice manual. Technical informa-tion bulletins also supplement theparts list until an update is issued.
Service manuals and all technicalinformation bulletins describingengineering changes are intended
exclusively for the use of STIHLservicing dealers. They must notbe passed to third parties.
Symbols are used in the text andillustrations for greater clarity.
The meanings are as follows:
In the descriptions:
= Action to be taken asshown in the illustration(above the text)
- = Action to be taken thatis not shown in theillustration(above the text)
In the illustrations:
= Pointer
= Direction of movement
Always use original STIHLreplacement parts.They can be identified by theSTIHL part number,
the lgo
and the STIHL parts symbol(
The symbol may appear alone onsmall parts.
1. Safety 2. IntroductionPrecautions
2 Carburetors
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3. Assignment
Model Carburetor Idle systemPartload
Fullload
Setting Equipment
depen
den
t
indepen
den
t
fixe
didlejet
pre
lim
inarym
ixturecon
tro
l
ba
lance
d
withbypassa
irs
crew
(LD)
part-l
oa
djet
full
-loa
dfixe
djet
full
-loa
dsem
i-fix
edjet
H
un
lim
ite
da
dju
stmen
t
Lun
lim
ite
da
djus
tmen
t
H,
Lw
ithlim
itercaps
cho
kes
hu
tter
in
carb
manua
lfue
lpum
p
Compensa
tor
009 ... 012 C1S-S1 x x x
009, 010, 011 WT-29A x x x
009, 011 C1S-S1C 2) x x x
009, 011 1) WT-323 x x x x
017 WT-325A x x x x
017 WT-325 x x x x019T C1Q-S37A x x x x x x
020 WT-15A x x x
020 C1Q-S32 x x x
020T C1Q-S16A x x x x x x
020T 1) WT-326C x x x x x
020T WT-326 x x x x
021 1) WT-503 x x x x x
021 WT-286A x x x x x
021, 023 1) WT-283A x x x x x
021, 023, 025 C1Q-S11 x x x x
023 WT-215 x x x x023 WT-283B x x x x x
023 WT-286A x x x x x
023C WT-286A x x x x x
023C WT-498 x x x x x x
023CZ 1) WT-498A x x x x x x
023L WT-360 x x x x x
023L 1) WT-396 x x x x x
024 WT-194 x x x
024 WT-110 x x x
024, 026 WT-194 x x x
025 WT-215 x x x x025 WT-286A x x x x x
025 WT-396A x x x x
025 WT-498A x x x x x x
025 WT-283B x x x x x
025 WT-313 x x x x x
026 WT-426 x x x x x
026 1) WT-493 x x x x
026 1) WT-403A x x x x x
026W/Pro WT-427 x x x x x
028 WT-16 x x x
Carburetors 3
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Model Carburetor Idle systemPartload
Fullload
Setting Equipment
depen
den
t
indepen
den
t
fixe
didlejet
pre
lim
inarym
ixturecon
tro
l
ba
lance
d
withbypassa
irscrew
(LD)
part-l
oa
djet
full
-loa
dfixe
djet
full
-loa
dsem
i-fix
edjet
H
un
lim
ite
da
dju
stmen
t
Lun
lim
ite
da
djus
tmen
t
H,
Lw
ithlim
iter
caps
cho
kes
hu
tter
in
carb
manua
lfue
lpum
p
Compensa
tor
029 HD-19B x x x x x x x
029 1) HD-18A x x x x x x
029, 039 HD-5 x x x x x x
034 C3A-S38A x x x x x x
034 C3A-S38 x x x x x x
034, 036 C3A-S19 x x x x036 C3A-S4C x x x x
036 C3A-S31D x x x x x x x
036 1) C3A-S39A x x x x x x
036W C3A-S27C x x x x x x x
038 HK-42 x x x x
038 Bing 48 x x x x
039 HD-19B x x x x x x x
039 1) HD-21A x x x x x x
044 HD-10 x x x x x x x
044 HD-11 x x x x x x x
044 HD-15 x x x x x x x044 C3M-S20 x x x x x x
044 C3M-S5G x x x x x
044 1) HD-17 x x x x x x
044 (SA) C3M-S21/S12 x x x x x x
046 HD-14 x x x x x x x
046 1) HD-16 x x x x x x
046 BR HD-24 x x x x x x x
051, 050, 076 HS-60D x x x x
056 HS-118B x x x x
056 WJ-4B x x x x
064 WJ-48 x x x x x x x064 WJ-51 x x x x x x x
064 WJ-52 x x x x x x x
064 WJ-65 x x x x x x x
064, 066 WJ-35/41 x x x x x x x
066 WJ-66 x x x x x x x
066 WJ-67 x x x x x x x
066 1) WJ-69 x x x x x x
066 BR WJ-76 x x x x x x x
066 MW WJ-66 x x x x x x x
070 LB-S9 x x x x
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Model Carburetor Idle systemPartload
Fullload
Setting Equipment
depen
den
t
indepen
den
t
fixe
didlejet
pre
lim
inarym
ixturecon
tro
l
ba
lance
d
withbypassa
irscrew
(LD)
part-l
oa
djet
full
-loa
dfixe
djet
full
-loa
dsem
i-fix
edjet
H
un
lim
ite
da
dju
stmen
t
Lun
lim
ite
da
djus
tmen
t
H,
Lw
ithlim
iter
caps
cho
kes
hu
tter
in
carb
manua
lfue
lpum
p
Compensa
tor
070 HL-324A x x x x
070, 090 HL-324A x x x x
08 HL-166C x x x x
084 HT-7A x x x x x x x
088 1) HT-12A x x x x x x
08S LA-S8A x x x x08S IVH8 x x x x
090 LB-S9 x x x x
BG 72 WT-253 x x x x
BG 72 1) WT-330 x x x
BG 75 C1Q-S30C x x x x x
BR 106 C1Q-SK6 x x x x x x
BR 320 1) HD-13A x x x
BR 320 HD-4B x x x x
BR 320 L WT-230 x x x x
BR 320 L WT-230B x x x x
BR 320 L 1) WT-489 x x xBR 320 L WT-331 x x x
BR 320, 400 1) HD-7 x x x
BR 400 1) HD-13A x x x
BR 400 HD-4B x x x x
BR/SR 320, 400 HD-4 x x x x
BT 106, 108 C1Q-SK5 x x x x x x
BT 360 HL-327E x x x x
FC 44 1) WT-327 x x x
FC 72 WT-227F x x x x
FC 72 1) WT-329 x x x
FR 106, 108 C1Q-SK5 x x x x x xFR 108 C1Q-SK5 x x x x x x
FR 108 1) C1Q-SK7 x x x x x
FS 106, 108 C1Q-SK5 x x x x x x
FS 120, 300 C1Q-S35B x x x x x x x x
FS 120, 300 C1Q-S36D x x x x x x x x
FS 160 C1S-S3D x x x x
FS 160, 180, 220 C1S-S3C x x x x
FS 200, 350 C1Q-S36D x x x x x x x x
FS 220 C1S-S3D x x x x
FS 25-4 WYL-63/-73 x x x x x
Carburetors 5
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Model Carburetor Idle systemPartload
Fullload
Setting Equipment
depen
den
t
indepen
den
t
fixe
didlejet
pre
lim
inarym
ixturecon
tro
l
ba
lance
d
withbypassa
irscrew
(LD)
part-l
oa
djet
full
-loa
dfixe
djet
full
-loa
dsem
i-fix
edjet
H
un
lim
ite
da
dju
stmen
t
Lun
lim
ite
da
djus
tmen
t
H,
Lw
ithlim
iter
caps
cho
kes
hu
tter
in
carb
manua
lfue
lpum
p
Compensa
tor
FS 280 C1S-S3D x x x x
FS 280 WT-223 x x x x
FS 36, 40, 44 WT-160B x x x x x
FS 36, 40, 44 1) WT-492A x x x x x
FS 36, 40, 44 WT-327 x x x x
FS 360, 420 HD-3 x x x xFS 400 C1Q-S34C x x x x x x x x
FS 450 C1Q-S34C x x x x x x x x
FS 500 HD-22 x x x x x
FS 500 HD-23 x x x x x
FS 550 1) HD-22 x x x x x
FS 550 HD-23 x x x x x x
FS 65-4 WYL-63 x x x x
FS 72, 74, 76 WT-227F x x x x
FS 72, 74, 76 1) WT-393 x x x
FS 75, 80, 85 C1Q-S41A x x x x x
FS 75, 80, 85 WT-447 x x x xFS 81, 86 2) WT-112 x x x x
FS 86 2) WT-45A x x x x
FS 88, FR 88 1) WT-367 x x x
FS 88, FR 88 2) WT-45A x x x x
FS/FR 108 1) C1Q-SK7 x x x x x
HS 72, 74, 76 WT-264 x x x x x
HS 72, 74, 76 1) WT-329 x x x
HS 75, 80, 85 C1Q-S42A x x x x x
TS 350 HL-292 x x x x
TS 350 HL-292G x x x x
TS 3501)
HL-366A x x x xTS 350 LAS6 x x x x
TS 360 HL-327D x x x x
TS 360 HL-327E x x x x
TS 360 LAS7 x x x x
TS 400 HS-274A x x x x x x x
TS 400 HS-274D x x x x x x x
TS 400 1) HS-279B x x x x x x
TS 460 HS-262B x x x x x x x
TS 460 1) HS-275A x x x x x x
TS 460 HS-276D x x x x x x x
6 Carburetors
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Model Carburetor Idle systemPartload
Fullload
Setting Equipment
depen
den
t
indepen
den
t
fixe
didlejet
pre
lim
inarym
ixturecon
tro
l
ba
lance
d
withbypassa
irscrew
(LD)
part-l
oa
djet
full
-loa
dfixe
djet
full
-loa
dsem
i-fix
edjet
H
un
lim
ite
da
dju
stmen
t
Lun
lim
ite
da
djus
tmen
t
H,
Lw
ithlim
iter
caps
cho
kes
hu
tter
in
carb
manua
lfue
lpum
p
Compensa
tor
TS 510 HS-277A x x x x x
TS 510 1) HS-280A x x x x x x
TS 510, 760 HS-212B x x x x
TS 760 HS-277A x x x x x
TS 760 1) HS-281A x x x x x x
1) CARB, EPA2) Accelerator pump
Carburetors 7
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The carburetor designation isstamped or printed on the bodyof some models (e.g. HT-7A orWT-223).
On some models the familydesignation (e.g. C3M or HD) isintegrally cast on the body.
Other identification numbers arestamped on the body of suchmodels (e.g. S 20) or printed (e.g.S36C).
The carburetor has to mix fuel andair in an optimum ratio for eachphase of engine operation. It pre-pares the mixture for vaporizationby breaking it down into finedroplets.
In the carburetor, fuel is atomizedbut not completely vaporized.The heat necessary to vaporizethe fuel is taken from the compo-nents the fuel flows through, i.e.primarily the engine. Therefore,complete vaporization does nottake place until the mixture rea-ches the combustion chamber.
The absorption of vaporizationheat by the fuel represents a formof cooling for the components that
give off the heat. One talks aboutinternal cooling.
The ideal fuel-air ratio (Lambda ()= 1) is reached when the composi-tion of the mixture is 14.8 kg air to1 kg fuel. Fuel burns cleanly at thismixture ratio.The mixture is combustiblebetween Lambda 0.7 and 1.25.
The mixture ratio deviates fromthe ideal value depending on the
prevailing operating conditions(temperature, humidity, air pre-ssure) and the operating phase(starting, idle, part load, full load).
When the actual mixture ratiodeviates from the ideal value, themixture is said to be either rich orlean.
A lean mixture contains more air.
As there is a lack of fuel, theengine does not produce its fullpower. Furthermore, the engineoverheats because insufficientheat is dissipated to vaporize thefuel. The lack of fresh gas contain-ing oil results in inadequate lubri-
cation and a risk of seizure.
A rich mixture contains less air.
Combustion is incomplete. Apartfrom other effects, unburned fuelis seen as smoke at the exhaust.
A certain amount of oil is addedto the fuel for lubrication of thebearings and piston.
3.1 Designation 4. Purpose of Carburetor
WT
223
HT 7A
915RA001VA
L H
L H
C3M
VA
915RA002
HD
VA
915RA003
S20
8 Carburetors
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A carburetor consists of a hollowhousing which has a specialinternal contour, i.e. the choketube (venturi) (1). Engine intakeair flows through this venturi. Thesmaller the cross section of theventuri, the greater the air flowspeed and the vacuum created.
Fuel jets open into the venturi atvarious points from which the air-stream draws fuel.
The fuel pump, nozzle or jet sys-tems and metering devices foroptimum preparation of the fuel-airmixture are integrated in ormounted on the carburetor.
The flow of air, and thus enginepower, is regulated by the throttleshutter (2).
In view of the machine applica-tions, the carburetor must operatetrouble-free in all positions. Forthis reason only all-positiondiaphragm carburetors with anintegral fuel pump are used inSTIHL outdoor power tools. Such
carburetors have no float.
The fuel pump operates as acompletely separate unit eventhough it is integrated in thecarburetor housing.
The fuel pump is controlled bythe changes of pressure in thecylinder and crankcase.
Intake process
The impulse chamber (1) is con-nected to the crankcase via theimpulse port (2).
Low pressure is created in thecrankcase by the upward stroke ofthe piston. This causes the pumpdiaphragm (3) to flex. A vacuum isalso created in the pumps fuelchamber (4).
Atmospheric pressure in the tanknow pushes fuel via the intakefitting (5) and inlet valve (6) intothe fuel chamber (4).
The outlet valve (7) is pressedagainst its seat and closes thepassage to the fuel port (8).
Note: The pump inlet and outletvalves are flaps stamped out ofthe pump diaphragm but stillattached to it at one side.
Delivery process
The downward stroke of the pistoncreates an overpressure whichpresses the pump diaphragm (3)
towards the fuel chamber.
5. Construction/ 5.1 Fuel PumpFunction
VA
915RA004
4
56
7
8
2 31
56
VA
915RA005
4
7
8
32 1
Carburetors 9
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The pump diaphragm thereforereduces the volume of the fuelchamber (4).
Inlet valve (6) is now pressedagainst its seat and closes thepassage to the intake fitting (5).
The outlet valve (7) is lifted off itsseat and allows a certain amountof fuel to flow into the fuel port (8)which leads to the carburetor inletvalve.
Note: During each complete pumpstroke an amount of fuel is deliver-ed which is equivalent to thedifference in volume between thetwo end positions of the pumpdiaphragm in the fuel chamber.
On machines with a fuel tankmounted lower than the carburetorit is necessary to prime the carbu-retor with fuel after a prolongedout-of-service period by pulling thestarter rope a few times.
This problem can be overcomewith a manual fuel pump (seebelow).
Inlet needle valve
The chamber (1) above themetering diaphragm (2) is filledwith fuel through port (3).
Atmospheric pressure is applied tothe metering diaphragm throughbore (4).If the machine is equipped with acompensator, the pressure prevail-ing at the clean side of the air filteracts on the metering diaphragm
(see chapter 4.3.4).
When air flows through theventuri, fuel is drawn out of themetering diaphragm chamber, i.e.the volume of fuel is reduced. Themetering diaphragm is pushedupwards by atmospheric pressure.
The metering diaphragm (2)applies pressure to the inlet lever(5) and thus lifts the inlet needle(6) off its seat. Fuel now flows
from the pumps fuel chamber tothe metering diaphragm chamberuntil the pressure in the meteringdiaphragm chamber is approxi-mately equal to the pressureapplied to the outside of themetering diaphragm.
The metering diaphragm alwaysallows a quantity of fuel to flowwhich is equal to that which isdrawn out of the jets.
Manual fuel pump
Some carburetors (on power tools)are equipped with an additionalmanual fuel pump for priming thefuel system prior to starting.
Note: On chainsaws the manualfuel pump is a separate compo-nent, i.e. it is not integrated in thecarburetor.
When the pump bulb (1) is de-pressed, the valve (2) opens andallows air or any fuel in the bulbto flow back to the tank via thefitting (3).
A vacuum is created in the meter-ing diaphragm chamber when thepump bulb is released. This cau-ses the metering diaphragm (1) tobe pulled against the inlet controllever (2). The inlet needle (3)opens and fuel then flows from thepump chamber into the meteringdiaphragm chamber.
Excess fuel pumped into themetering diaphragm chamberpasses through the bore (4) to the
valve. The valve tabs open andfuel flows into the pump bulb.When the pump bulb is pressed,fuel is pumped back into the tankvia valve (2).
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Check valve
A check valve is installed in themetering diaphragm chamber ofall carburetors equipped with anadditional manual fuel pump.
When the manual fuel pump isoperated, the check valve (1) clo-ses to prevent air being suckedinto the metering diaphragm cham-ber from the venturi and the idle
jet bores. During operation thecheck valve opens and allows fuelto flow to the main jet and the idle
jet bores.
Full load is the operating conditionwith the throttle shutter wide open.
Fuel is drawn from the main jet (1)and the idle jets (2 and 3).
Main jet (valve jet)
The main jet (1) is situated at thenarrowest point of the venturi. It isat this point that air flow speed ishighest and pressure is lowest.
The main jet is equipped with acheck valve (2) to prevent airbeing sucked into the meteringdiaphragm chamber at idle speed.
The amount of fuel drawn in by the
main jet can be adjusted with thehigh speed screw (3).
Full-load semi-fixed jet
At full load, a defined quantity offuel (approx. 80% at H = 1) is al-ways pumped from the metering di-aphragm chamber (3) via the full-load semi-fixed jet (1) and throughthe main jet (4) to the venturi, irre-
spective of the setting of the highspeed screw (2). This means thatthe change in the composition ofthe mixture for a given adjustmentof the high speed screw is far lessthan on a carburetor that has nofull-load semi-fixed jet.
The semi-fixed jet may be installedas a separate component or beintegrated as a nozzle bore in themain jet. There is no direct connec-tion between the separate semi-
fixed jet and the venturi. It is con-nected to the main jet via apassage (5).
Full-load fixed jet
On carburetors with a full-loadfixed jet (1) all the fuel required forfull-load operation flows via this
jet.
5.2 Operating Conditions5.2.1 Full Load
VA
915RA009
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There is no high speed screw. Thefull-load setting cannot be altered.
Like the semi-fixed jet, the full-loadfixed jet may be installed as aseparate component or be inte-grated as a nozzle bore (2) in the
main jet. There is no direct connec-tion between the separate fixed jetand the venturi. It is connected tothe main jet via a passage (3).
The throttle shutter (1) alters thecross section of the venturi andthus regulates the flow of intakeair.
It is operated by the user via thethrottle rod.
The idle position of the throttleshutter is adjusted by means ofthe idle speed screw.The throttle shutter is almostclosed. Air flows through a narrowgap between the throttle shutterand venturi. Fuel is drawn fromthe primary idle jet (2).There is only a slight vacuum inthe area of the main jet (3) so nofuel flows at this stage.
Idle jets - bypass bores
There are two or three idle boresin the area of the throttle shutter.
When the throttle shutter is closedthere are two bores (secondaryidle or bypass) (1 and 2) in front ofit and one (primary idle) (3) behindit (viewed in the flow direction).
In this position the air flow in the
venturi is very slow and the vacu-um is slight. Fuel is drawn onlyfrom the primary idle jet at the airgap between the throttle shutterand carburetor body.
As a result of the difference inpressures (pressure in front ofthrottle shutter is close to atmos-pheric pressure), air from theventuri passes through thesecondary idle jets (bypass bores)(1 and 2) into the idle chamber (4).
This air mixes with the fuel and isdrawn out of the primary idle jet(3) as a fuel-air mixture at the lowpressure side.
In this process more air is addedto the relatively small amount ofair which flows through the smallnotch or hole in the throttle shutterand only the quantity of fuel re-quired for idling is drawn off.
When the throttle shutter isopened the vacuum acts on thearea of the secondary idle jets(1 and 2). Fuel is then drawnfrom these bores as well.
5.2.2 Idle
915RA184
4
2
BL
31
915RA185
53
BL
21
4
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As the quantity of air that flowsthrough the venturi is now greater,this fuel is necessary to enrich themixture during acceleration.
The amount of fuel drawn fromidle jets (1, 2 and 3) can be in-creased or decreased by means
of the low speed screw (5) whosepointed tip varies the size of thepassage to the idle chamber (4).
Idle system independent ofmain jet system
In these carburetors the main jetand idle systems are arranged inparallel.Changes in the idle fuel volumemean a change in the full-load fuel
volume. If the idle fuel volume isaltered with the low speed screw(2), it is necessary to readjust thehigh speed screw (1).
When the idle setting is madeleaner, the overall fuel-air mixturebecomes leaner too. This in-creases engine speed as well asthe risk of engine damage as aresult of overheating.
Idle system dependent onmain jet system
In these carburetors the idlesystem branches off the full-loadsystem after the high speed screw(1). It is not directly connected tothe metering diaphragm chamber.
Full-load fuel flow is determinedby the high speed screw.
Adjusting the low speed screw (2)to change the idle fuel volumedoes not result in a change to theoverall fuel volume.
915RA185
53
BL
21
4
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Idle
In the idle positon, the throttleshutter is situated between the2nd secondary idle jet (5) and theprimary idle jet (6).
The pressure in front of the throttle
shutter is close to atmosphericpressure, while the vacuumcreated by the engine prevailsbehind the throttle shutter.Vacuum acts on the idle chamber(3) via the primary idle jet (6). As aresult of the pressure differential,air flows from the venturi throughthe secondary idle jets (4 and 5)into the idle chamber (3).At the same time, fuel is drawnout of the metering diaphragmchamber (1) and into the idle
chamber (3) via the fixed idle jet(2).A primary mixture is created inthe idle chamber (3) which issucked through the primary idlebore (6) into the area at the engineside of the throttle shutter.
The idle air is drawn through anotch or a hole in the throttleshutter and mixes with the primarymixture to form the idle mixture.
The proportion of primary mixturein the idle mixture can be adjustedwith the idle screw (7) (primarymixture control).The machine will run with thebasic setting (approx. 1 turn open).The idle mixture is made richer by
opening the idle screw (7) orleaner by closing the screw.If necessary, the idle speed canbe adjusted with the idle speedscrew (LA).
Transition from idle topart load
When the throttle shutter isopened, vacuum acts on thesecondary idle jets (4 and 5).Fuel is then drawn from all three
idle jets (4, 5 and 6).
Transition from part load tofull load
The main jet begins to deliver fuelwhen the throttle shutter hasopened about 20 degrees. Theidle and full-load system deliverthe optimum amount of fuel to suitgiven opening conditions when thethrottle shutter is opened fully.
Advantages over conventionalcarburetors
The fixed idle jet (2) limits enrich-ment for acceleration and theamount of fuel that flows via theidle system during full-load opera-tion. As a result, the setting ofthe idle screw (7) has no effectat all on the volume of fuel atfull load.
Reduced sensitivityto temperature
Carburetors with fixed idle jetsensure a much steadier idlingbehavior at low temperatures:Fuel becomes viscous at lower
temperatures.
The more viscous the fuel is, thegreater the influence the shape ofthe throttle orifice has on the flowrate.Conventional adjusting screwscreate an annular gap (1) at thethrottle orifice which can becomeblocked within a relatively shorttime.Fixed jets have a round cross sec-tion (2) at the throttle orifice whichallows a constant flow of fuel evenat low temperatures. A constant
fuel flow rate is the preconditionfor a steady idling behavior.
5.2.3 Carburetor with Fixed Idle Jet and Primary Mixture Control
VA
915RA292
2
1
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The graph shows the air-fuel ratio
(Lambda ) or the carbon mon-oxide content (CO) in the exhaustgas plotted against the idle RPM:
Carburetors with a balanced idlesystem (BIS) guarantee an idealidle mixture at all idle speeds.
Benefits
These carburetors arecharacterized by:- Smooth, uniform idle speed- Good response and acceleration- Lower emissions under all
operating conditions- Stable adjustment over a wide
ambient air temperature range- Only one adjusting screw
makes adjustment simpler.
Special features
A fixed idle jet and an air bypasssystem with calibrated bores aretypical features of carburetors witha balanced idle system.
The throttle shutter is completelyclosed in the idle position.
Most of the air required to form theidle mixture flows through the holein the throttle shutter.The air required to adjust the idlespeed is sucked in via the idlechamber - or air bypass port.
The amount of bypass air can beadjusted with the idle speed screw(LD screw).
Low engine idle speed,the LD screw restricts theair bypass port:
The vacuum Pu behind thecompletely closed throttleshutter (7) causes -- air to flow through the hole (8) in
the throttle shutter- vacuum P3 in idle chamber (3)
via the primary idle bore (6)- vacuum P11 in air bypass cham-
ber (11) via the bypass port (10).
If pressure P11 in air bypasschamber (11) is -- lower than the atmospheric
pressure in front of throttleshutter (7), air flows through by-pass port (9) into the air bypasschamber (11)
- greater that pressure P3 in idlechamber (3), air flows from by-pass chamber (11) through theconnecting passage (4) to theidle chamber (3).
The idle chamber (3) receives airvia the secondary idle bore (5) aswell as the connecting passage (4).At the same time, fuel is drawn outof the metering diaphragm cham-ber (1) via the fixed idle jet (2).
A primary mixture is formed in theidle chamber (3) which is suckedthrough the primary idle bore (6)into the engine side of the throttleshutter.
Primary mixture, air from the by-pass port (10) and the air whichflows through the hole (8) in thethrottle shutter mix together toform the idle mixture.
Higher engine idle speed
The LD screw is turned clockwiseto increase idle speed.
Note: The LD screw has a left-hand thread in order to retain thefamiliar directions of rotation foradjustments:- Rotating the screw clockwise
increases engine speed.- Rotating the screw counterclock-
wise reduces engine speed.
Turning the LD screw clockwisereduces the pressure P11 in theair bypass chamber (11). Thisincreases the air throughput inbypass port (10). The differencebetween P3 and P11 becomessmaller so that less air flowsthrough the connecting passage(4) to the idle chamber.
5.2.4 Carburetor with Balanced Idle System(Fixed Idle Jet and Air Bypass System)
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The flow of air in connectingpassage (4) stops when P3 andP11 are equal.
Turning the LD screw furtherclockwise reverses the pressuregradient. P11 is then lower than P3so that primary mixture from theidle chamber (3) is added to thebypass air flowing through bypassports (9 and 10) via the connectingpassage (4).
Part load means all stages ofengine operation with a partlyopened throttle shutter.
Fuel is drawn from the primaryidle jet (1) and the secondaryidle jet (2).
Part-load fixed jet
When working in the part-loadrange (e.g. limbing) the throttleshutter is often in a position be-tween idle and full throttle. In thissituation the fuel-air mixture maybecome over-lean ("part-load flatspot") and there is then a dangerof seizure.
The part-load fixed jet (1), whichopens into the venturi between theidle jets and the main jet, preventsthe mixture becoming over-lean.
The low pressure causes addi-tional fuel to be drawn off andenrich the fuel-air mixture inresponse to the throttle shutterposition.
5.2.5 Part Load
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Hot startCompared to engine idle RPM, thespeed achieved by cranking theengine on the starter is very low.The gas flow speed and the vacu-um in the venturi are thereforelimited. Fuel and air mix poorly in
this situation.Moreover, fuel is more sluggishthan air. This means than consider-ably more air than fuel is suckedin, the mixture becomes too leanand does not burn well.
Cold startDuring a cold start the problemsdescribed for a hot start are furtheramplified.Cold components cannot transferheat to the mixture. As a result,
the mixture gasifies poorly anda large proportion of the fuelcondenses on the walls of thecomponents it flows through.
The mixture in the carburetorhas to be highly enriched for thecold start: the choke shutter (1)is closed.
The full vacuum acts on the idlejets (3 and 4) and main jet system(2) in the metering diaphragmchamber. Fuel is drawn in and mi-xed only with the air that flowsthrough the notch or hole in thechoke shutter.
As soon as the engine has fired itis necessary to make the mixtureleaner, i.e. open the choke shutter.The heat generated by initial com-bustion evaporates part of the fuelthat had previously settled on thecomponent walls. This has anenriching effect while the engineis warming up.
Note: Once the engine is running,open the choke shutter immedi-
ately to avoid an over-rich fuel-airmixture (the engine would other-wise stall again).
Automatic choke
Some machine versions areequipped with an automaticchoke. It sets the throttle andchoke shutters to the correctpositions for an optimum mixture.Choke shutter and throttle shutter
with bellows and control valve areconnected via a lever mechanismon the carburetor.
5.3 Problematic Operating Conditions5.3.1 Starting
VA
915RA02
6
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The automatic choke uses cylindertemperature as a parameter.Cross section "A" is changed as afunction of cylinder temperature.This, in turn, alters the time re-quired to vent the bellows andthus the opening speed of thechoke shutter.
The regular changes in pressurewhich occur in the crankcase areused to operate the automaticchoke.
These pressure waves are fed viaa hole in the crankcase andthrough a port in the cylinder tothe control valve (1).
The control valve directs the lowpressure waves at a predeter-mined interval to the bellows (2)which contract and thus open thechoke shutter (3).
The period for which the bellowsopen the choke shutter dependson cylinder temperature. It variesbetween 3.5 seconds on a coldengine to approx. 0.1 seconds ona hot engine.
A lever mechanism connectsthe choke shutter to the throttleshutter.
The throttle shutter (1) is at anangle of 37 degrees to thecarburetor mounting flange justbefore the choke shutter (2) opensfully. The throttle shutter returns tothe idle position when the chokeshutter is fully open.
Following a starting attempt, thebellows are filled with air from thecarburetor box and returned to theneutral position. The air requiredto pressurize the bellows is drawnin from the carburetor box via afoam filter, a polymer sinteredfilter, a hose and control valve.
The pressurizing process isaccompanied by a low whistlingnoise.
A depression is maintained inthe bellows while the engine isrunning, i.e. the bellows arecontracted; the choke shutter isopen.
VA
915RA023
VA
915RA027
37
1 2
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Icing can occur in the area of theintake if humidity is high and tem-peratures are below + 10C (50F).
Cause:Air can absorb a certain amount ofmoisture. The higher the air tem-
perature the greater its capacity toabsorb moisture.
If the engine draws in damp coldair, pressure and temperaturedrop and so does the ability to ab-sorb moisture. Moisture conden-ses on the intake passages, espe-cially in the carburetor. Thisresults in ice forming, which blocksthe jets and causes engine run-ning problems.
If the air filter is dirty, the air flowrate and pressure in the venturi(Pi) drop.
Carburetor heating
Some models are equipped withan electric carburetor heatingsystem for operation in extremelycold climates, in which the intakeair from around the engine is stilltoo cold.
The heating element (1) is posi-tioned on the carburetor mountingstuds.
The thermostatic switch (1) islocated on the pump end cover.Power supply takes place automat-ically via the heating generator.
Accelerator pump
Some carburetors are equippedwith an accelerator pump. Thepiston (1) is located in a borebehind the throttle shaft (2) andis held in its neutral position by aspring (3).
When the throttle shutter isopened quickly for acceleration,the air is able to adapt rapidly tothe new flow conditions becauseof its low mass.
The reaction of the fuel, however,is sluggish by comparisonbecause it first has to be set inmotion by the vacuum. Thismeans that a certain time lapsesbefore the fuel has also adapted to
the new conditions in the venturi.A flat spot would normally occurbecause of the over-lean fuel-airmixture.
The accelerator pump offsets thiseffect.
5.3.2 Temperatures/Humidity 5.3.3 Acceleration
VA
915RA291
1
VA
915RA029
3
2
1
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The accelerator pumps piston (1)is located in a bore behind thethrottle shaft. The space below thepiston is filled with fuel.
When the throttle is opened, thecontrol face on the throttle shaftpushes the piston inwards. The
fuel below the piston is theninjected into the venturi via thevalve jet (3).
This enriches the fuel-air mixtureto ensure a smooth transitionwithout a flat spot during accelera-tion. When the throttle shaft isturned back, the spring (2) returnsthe pump piston to its neutral posi-tion. A vacuum is created andfresh fuel is drawn from the meter-ing system into the space below
the piston. Compensator
1 Air filter2 Carburetor3 Valve jet4 Cylinder5 Metering diaphragm6 Inlet control valve7 Fuel pump8 Fuel tank9 CompensatorPa Atmospheric pressurePi Internal pressure
Pr Pressure in meteringchamber
The compensator connects theclean side of the air filter to thefuel metering system in thecarburetor. This means that thepressure applied to the meteringdiaphragm is identical to theinternal pressure Pi in the intake -not atmospheric pressure Pa.
In this way fuel feed is dependenton the current internal pressure Piand the amount of air or oxygenthat is actually available.
The air flow volume is reducedwhen the air filter is dirty. Thiscauses the pressure Pi at theclean side of the air filter to drop,and also the pressure appliedto the outside of the metering dia-phragm. The metering diaphragmmoves outwards and the inlet
control valve reduces the crosssection.
The flow of fuel is reduced sothat the mixture cannot becomeover-rich.
5.3.4 Dirty Air Filter/Compensator
VA
915RA030
3
2
1
7
3
1
9
2
5
6
8
4P
915RA031VA
a Pi
Pi
Pr
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A carburetor equipped with a com-pensator therefore keeps the fuelcontent in the mixture constant atall times, irrespective of air filtercontamination.It is no longer necessary to alterthe setting of the high speed
screw as contamination of theair filter increases.
However, engine power drops asa result of the reduction in airvolume and can only be restoredto normal by cleaning the air filter.There is no need for any furtheradjustment after cleaning.
A conversion from a HD (textile)filter to a standard (wire mesh)filter or vice versa does not neces-
sitate readjustment of the mixtureeither.
Power curve
P = Powern = RPMa = C lean air filterb = dirty air filter,
with compensatorc = dirty air filter,
without compensator
The mixture cannot become over-rich. And it is no longer necessaryto alter the setting of the highspeed screw as contamination ofthe air filter increases. However,engine power drops due to thereduced quantity of air and fuel.Optimum power can only be resto-red by cleaning the air filter.
Control valve
Carburetors that are directlyflange-mounted to the cylinder(without a resilient manifold toinsulate vibrations) may beequipped with a control valve.
This valve (1) opens in responseto resonance at a predeterminedengine speed. Additional fuel isdrawn into the venturi through thepassage (2) and the outlet bore.The fuel-air mixture is enriched sothat the engine speed cannotincrease any further.
5.4 Special Features
915RA298VA
VA
915RA033
2 1
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Speed governor (vane)
A torsion spring (1) acts via thegovernor lever and rod to keepthe vane (2) in its neutral position(choke shutter is open). As enginespeed increases, the force of thecooling airstrean overcomes
spring tension and deflects thegovernor vane. This movement istransmitted to the choke shutter.
When a predetermined limit RPMis reached, the governor vane isdeflected to a maximum degreeand the choke shutter closes to apoint where the richer mixtureprevents any further increased inRPM.
Note: The cut-off speed can be
adjusted by changing the springspreload.
VA
915RA034
1
2
22 Carburetors
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Condition Cause Remedy
Carburetor floods; Inlet needle not sealing. Remove and clean or replaceengine stalls Foreign matter in valve seat inlet needle, clean fuel tank,
or cone damaged pickup body and fuel line ifnecessary
Inlet control lever sticking on Free off inlet control leverspindle
Helical spring not located on Remove inlet control levernipple of inlet control lever and refit correctly
Perforated disc on diaphragm Fit new metering diaphragmis deformed and pressesconstantly against inlet controllever
Inlet control lever too high Set inlet control lever flush(relative to design position) with top of body or bottom
of metering chamber
Poor acceleration Idle jet "too lean" Back off low speed screwslightly
Main jet "too lean" Back off high speed screwslightly
Inlet control lever too low Set inlet control lever flush(relative to design position) with top of body or bottom
of metering chamber
Inlet needle sticking to valve Remove inlet needle, cleanseat and refit
Connecting bore to atmosphere Clean boreblocked
Diaphragm gasket leaking Fit new diaphragm gasket
Metering diaphragm damaged Fit new metering diaphragmor shrunk
6. Troubleshooting Chart
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Condition Cause Remedy
Engine will not idle, Throttle shutter opened too wide Reset idle speed screwidle speed too high by idle speed screw correctly
Idle speed screw (LD) opened Adjust idle speed screw correctlytoo far
Idle speed screw (LA) opened Adjust idle speed screw correctlytoo far
Machine leaking Locate and repair leak
Engine stalls at idle speed Idle jet bores or ports Clean jet bores and portsblocked with compressed air
Idle jet "too rich" Screw down low speed screwslightly
Setting of idle speed Set idle speed screwscrew incorrect correctly- throttle shutter completely
closed
Setting of idle speed Set idle speed screwscrew (LD) incorrect correctly
Small plastic plate in valve Clean or renew valve jetjet does not close
Engine speed drops quickly Air filter plugged Clean air filterunder load - low power
Tank vent faulty Clean or replace tank ventif necessary
Leak in fuel line between Seal or renew connectionstank and fuel pump and fuel line
Pump diaphragm damaged Fit new pump diaphragmor fatigued
Main jet bores or ports Clean bores and portsblocked
Fuel pickup body dirty Clean pickup body, fit newfilter
Fuel strainers dirty Clean fuel strainers
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The carburetor can be testedfor leaks with the carburetorand crankcase tester (1)1106 850 2905.
- Check the tester for leaks.
The following test values refer
to carburetors that have beenflushed with fuel and havemoist diaphragms. Dry carbure-tors must not be subjected topressures of more than 0.35 bar(5 psi).
Flush dry carburetors with fuelbefore starting the test.
Carburetors can only seal properlywhen the fuel passages and allsealing points are wetted with fuel.
The pressure applied duringthe test must not exceed 1 bar(14.5 psi).
If higher pressures are applied,the control valve will open andallow the pressure to enter themetering chamber and possiblyover-stretch the meteringdiaphragm.
Push fuel line (2) 1110 141 8600
with nipple (3) 0000 855 9200onto the carburetors intakefitting.
Connect testers pressure hoseto nipple.
Close vent screw (4) on therubber bulb (5) and pump air intothe carburetor until the pressuregauge shows a reading ofapprox. 0.8 bar (11.6 psi).
If this pressure remains constant,the carburetor is airtight. However,if it drops, the likely causes are:
The inlet needle is not sealing(foreign matter in valve seat orsealing cone of inlet needle is
damaged or inlet control leversticking).
Check the metering diaphragm fordistortion and replace if necessary.
In these cases the carburetor mustbe serviced.
- After completing the test, openthe vent screw and remove thefuel line from the intake fitting.
Carburetor problems are oftencaused by blocked jets, bores andfuel strainers. Even water droplets,which get into the carburetor withthe fuel, can cause trouble. Owingto its high surface tension, waterrestricts the flow of fuel in the fine
jet bores.
Water droplets may freeze inwinter and completely block fuelpassages.
To clean the carburetor, strip itdown and remove the adjustingscrews, jets and inlet needle. Usea brush or a clean leather clothand fresh fuel to clean the carbure-tor body. Then blow out all portsand jets with compressed air.
Warning: Never use needles,wires or similar tools or fibrouscloths for cleaning the carburetoror jets.
7. Servicing7.1 Leakage Test 7.2 Cleaning
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Take out the screws.
Remove the end cover.
Remove the gasket (1) andpump diaphragm (2) from theend cover or carburetor body.
Note: If the gasket and diaphragmare stuck, remove them carefully.
Carefully separate thediaphragm and gasket.
- Inspect diaphragm and gasketand replace if necessary.
Note: The diaphragm and the inletand outlet valves are exposed tocontinuous alternating stressesand the material eventually shows
signs of fatigue. The diaphragmdistorts and swells and has to bereplaced.
Place the gasket (1) on the endcover (2).
Wet the pump diaphragm (3)with fuel and place it on thegasket.
- Fit the end cover on thecarburetor body.
Note: Pump diaphragm, gasketand end cover are held in positionby the integrally cast pegs on the
end cover.
- Insert screw(s) and tighten downsecurely.
Take out the screws.
- Remove the end cover.
- For carburetors with manual fuelpump see 7.9.
Remove metering diaphragmand gasket from carburetor bodyor end cover.
Note: If the gasket and diaphragmare stuck, remove them carefully.
Carefully separate thediaphragm and gasket.
7.3 Pump Diaphragm 7.4 Metering Diaphragm
VA
915RA039
3
1
2
VA
915RA036
VA
915RA037
21
VA
915RA038
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- Inspect diaphragm and gasketand replace if necessary.
Note: The diaphragm is exposedto continuous alternating stressesand the material eventually showssigns of fatigue. The diaphragm
distorts and swells and has to bereplaced.
Place gasket on carburetor body.
Place metering diaphragm ongasket.
Note: Gasket and metering dia-phragm are held in position by the
integrally cast pegs.
Fit pump diaphragm so thatperforated plate (1) pointstowards the inlet controllever (2).
If the peg (1) is on the end cover,place the metering diaphragm(2) and gasket (3) on the endcover.
- Fit end cover in position.
Carburetors with compensator: Install end cover with connector.
Fit screws and tighten downsecurely.
- Remove metering diaphragm -see 7.4.
Take out the screw.
- Remove inlet control lever withspindle or plate with inlet controllever and gasket. Pull out the
inlet needle.
If there is an annular indentationon the sealing cone of the inletneedle, it will be necessary toreplace the inlet needle becauseit will no longer seal properly.
7.5 Inlet Needle
VA
915RA045
1
2
VA
915RA048
VA
915RA046
2
1
3
VA
915RA047
VA
915RA043
VA
915RA044
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Fit the inlet needle (1).
Fit helical spring (2) in the blindhole.
Insert spindle (3) in the inletcontrol lever (4).
- Engage clevis of inlet controllever in annular groove on thehead of the inlet needle.
- Press down the inlet controllever and secure it with thescrew.
Note: Make sure that the helicalspring locates on the control lever-s nipple.
- Check easy action of the inlet
control lever.
Important: The top of the inletcontrol lever must be level with thebottom of the metering chamber(1) or the top face of the carbure-tor body (2).
- If this is not the case, the inletcontrol lever is bent and must bereplaced.
- Install metering diaphragm -
see 7.4.
If the plate of the check valve inthe main jet no longer movesfreely, install a new main jet.
- Remove metering diaphragm -see 7.4.
Use a suitable pin punch topress the main jet out of its seatin the direction of the venturi.
WS-26A carburetor:Use a screwdriver to push the
valve jet out of its seat frominside the venturi.
- Wash the main jet in white spirit.
Install the main jet exactlyvertical in the bore. Press ithome until it is flush with thebottom of the metering chamber.
7.6 Main Jet
VA
915RA058
VA
915RA05
6
VA
915RA05
4
1 2
VA
915RA057
VA
915RA051
2
1
3
4
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- Remove metering diaphragm -see 7.4.
Note: The part-load fixed jet (1)cannot be replaced, it can only beblown clear with compressed air.
Use a pin punch to press the full-
load fixed jet (2) out of its seat inthe direction of the venturi.
HS-262B carburetor: Unscrew the full-load fixed jet.
HD-5 carburetor: Use a scriber to remove the
retaining ring (1).
Tap the carburetor body againsta wooden base to remove thefull-load fixed jet (2) and 0-ring(3).
- Wash full-load fixed jet in whitespirit and blow out with compres-sed air.
Note: Replace damaged full-loadfixed jet.
- Install full-load fixed jet exactlyvertical in the bore. Press ithome until it is flush with thebottom of the metering chamber.
HD-5 carburetor:- Fit a new 0-ring in the bore.
- Press home the fixed jet (flatside up) as far as stop andsecure in position with retainingring.
- Install metering diaphragm -see 7.4.
WT-227E and WT-264carburetors:- Remove the manual fuel pump -
see 7.9.
Unscrew the control valve (1).
Take the copper washer (2) out
of the bore (3).
- Fit a new copper washer in thebore for the control valve.
- Screw in new control valve about2 full turns.
- Coat the thread that still projectsfrom the carburetor body withLoctite - see 8.2.
- Carefully screw home control
valve as far as stop and tightendown to 4 Nm (3.0 lbf.ft).
7.7 Full-Load Fixed Jet/ 7.8 Control ValvePart-Load Fixed Jet
VA
915RA060
2
1
VA
915RA062
3
2
1
VA
915RA063
123
VA
915RA061
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Take out screws (1).
Remove end cover with pumpbulb (2).
Remove the pump bulb.
Remove the flange.
Take valve out of flange.
- Check valve and replace ifnecessary.
Note: If the circuit for filling themetering chamber is malfunction-ing, if the valve is dirty, distorted or
damaged, install a new valve.
Check metering diaphragm andsealing ring and replace ifnecessary - see 7.4.
Assemble in the reverse sequence.
- Remove metering diaphragm -see 7.4.
Fit a 2.5x13 mm self-tappingscrew in the check valves boreand use it to pull out the checkvalve.
Use a 4 mm (approx. 5/32") pinpunch to press the check valvesquarely into the meteringchamber as far as stop.
- Install metering diaphragm -see 7.4.
7.9 Manual Fuel Pump 7.10 Check Valve
VA
915RA064
2
1
1
VA
915RA067
VA
915RA069
VA
915RA065
VA
915RA068
VA
915RA070
VA
915RA066
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- Remove pump diaphragm -see7.3.
- Remove inlet needle - see 7.5.
- Remove valve jet - see 7.6.
- Remove full-load fixed jet, iffitted - see 7.7.
- Remove check valve, if fitted -see 7.10.
Use a scriber to pry out fuelstrainer and then clean it.
Important: Always replace adamaged fuel strainer.
Take out the high speed screw(1), low speed screw (2) and, iffitted, the idle speed screw (3).
Caution: Idle speed screw (LD)has a left-hand thread.
Use a pin punch to press thecenter of the sealing plug (1) orsealing plate (2) until it bucklesand can be taken out.
Important: The sealing plug orsealing plate is destroyed duringremoval.
- Wash the carburetor body infresh white spirit and blow allports and bores clear withcompressed air.
- To replace the carburetor body,remove the throttle shaft and, if
fitted, the choke shaft - see 7.12and 7.13.
- Place the new sealing plug orplate in the bore with its convexside facing up.
- Use a large pin punch to applylight pressure until the sealingplug or plate is flat.
- Secure new sealing plate orplug with Loctite - see 8.2.
(Apply Loctite to gap betweencarburetor body and sealingplate or plug.)
Assemble all other parts in thereverse sequence.
7.11 Carburetor Body
VA
915RA071
VA
915RA074
2 1
3
VA
915RA075
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- For carburetors with automaticchoke see 7.15.1.
Take out the screw.
- Remove the throttle shutter orpull it out of the throttle shaft.
On C1Q-S11G, detach torsionspring (1) from groove in throttleshaft (2).
Remove the E-clip.
- Take off the washer, if fitted.
On some carburetors, removethe screw from the lever.
- Remove the lever.
Remove spacer sleeve.
On some carburetors, take outthe screw (1).
Pull the clip (2) off the throttleshaft.
Carefully withdraw the throttleshaft.
- Check for excessive play whichmay be a source of secondaryair.
- For carburetors with accelerator
pump see 7.14.
- On some carburetors it isnecessary to remove the pumpend cover before withdrawingthe throttle shaft - see 7.3.
Pull off the torsion spring.
7.12 Throttle S haft
VA
915RA077
VA
915RA080
VA
915RA078
2
1
VA
915RA081
VA
915RA079
VA
915RA085
2
1
VA
915RA086
VA
915RA087
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On throttle shafts with a slottedpin (1), remove the E-clip (2).
Take slotted pin out of thelever (3).
After pushing home the throttleshaft, check that torsion springis correctly positioned.
Note: The illustration shows theWT-227F carburetor. The positionof the torsion springs on othercarburetors is similar.
- Install throttle shutter so that itsentire circumference locatesagainst the wall of the venturi.
- Coat screw with Loctite, see 8.2,and tighten down securely.
- Fit E-clip or clip in the groove.
- Secure clip with screw.
On C1Q-S11G, attach torsionspring (1) to groove in throttleshaft (2).
On WT-230, push spacer sleeve(1) into position.
Fit lever (2) so that the slottedpin (3) points towards meteringchamber end cover (4).
On HD-3A, fit the spacersleeve (1).
Fit lever (2) so that slotted pin (3)points towards intake fitting (4).
On WT-253, fit lever (1) so that itpoints towards intake fitting (2).
On C1Q-SK6, fit lever (1) so thatslotted pin (2) points towards themetering chamber end cover (3).
VA
915RA088
22
1
3
VA
915RA090
2
1
VA
915RA089
4
3
2
915RA091VA
1
VA
915RA092
1
23
4
VA
915RA093
2
1
VA
915RA094
1
2
3
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- For carburetors with automaticchoke see 7.15.1.
Pull off the lever, if fitted.
On C1Q-S11G, detach torsionspring (1) from groove in throttleshaft (2).
Take out the screw.
- Remove the choke shutter or pullit out of the choke shaft.
Remove the E-clip, if fitted.
Withdraw the choke shaft withtorsion spring.
Carefully withdraw choke shaftwithout torsion spring so thatthe ball does not pop out and belost.
- Take out the ball and spring.
Remove the torsion spring.
- Fit the torsion spring.
After fitting the choke shaft,check that the torsion spring iscorrectly positioned.
7.13 Choke Shaft
VA
915RA099
VA
915RA078
2
1
VA
915RA101
915RA102VA
VA
915RA103
VA
915RA104
VA
915RA105
VA
915RA106
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On choke shafts without torsionspring, fit the spring (1) and ball(2).
Push home the choke shaft untilthe ball engages the groove (3).
- Install choke shutter so that its
entire circumference locatesagainst the wall of the venturi.
- Fit screw with Loctite, see 8.2,and tighten down securely.
- On choke shafts with torsionspring, fit the E-clip.
On C1Q-S11G, attach torsionspring (1) to groove in throttleshaft (2).
- Refit the lever, if removed.
- Remove the throttle shaft -see7.12.
Knock the pump piston (1) andspring (2) out of the bore (3).
Take the sealing ring (4) off thepump piston.
- Inspect pump piston and springand replace if necessary.
Fit new sealing ring (4) in grooveof pump piston (1).
Place spring (2) in the bore (3).
Fit pump piston (1) in the bore,closed end facing outward.
- Install throttle shaft - see 7.12.
- Press down the pump piston sothat throttle shaft can be pushedhome.
VA
915RA090
2
1
7.14 Accelerator Pump
3
VA
915RA107
3
11
2 2
VA
915RA109
3
2
4
1
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Removal:
Carefully pry bell crank "G" andlever "H" off the shafts.
- Remove the lever system fromthe shafts.
- Take the torsion spring off thethrottle shaft.
Installation:
Note: Bell crank "G" and lever "H"have to be pressed onto theshafts. To do this, clamp the car-buretor body in position so that thepressure used to install the leversis transmitted to the carburetorbody via a short part of the shaftand the retaining ring. This pro-
cedure ensures that the shaft isnot loaded beyond its bucklingstrength.
Carefully push the link "C" intoposition to connect levers "A"and "B".
Fit the torsion spring "D" on thethrottle shaft "E" so that its bentend points away from thecarburetor.
Push lever "A" on to the throttleshaft and, at the same time,lever "B" on to the choke shaft.
Rotate lever "B" on choke shaftcounterclockwise as far as stop.
Fit the bell crank "G" (with flatface pointing to lever "B") overthe choke shaft so that the pinon lever "B" is between the armsof bell crank "G".
Rotate bell crank counterclock-wise until its right arm butts
against the pin on lever "B".
With the bell crank in this posi-tion, carefully press it on to thechoke shaft.
Caution: Take care not to bend
the choke shaft.
VA
915RA110
H
G
VA
915RA112
E D
VA
915RA11
3
A
B
VA
915RA11
1
AC
B
VA
915RA114
B
7.15 Lever Mechanism on Carburetorswith Automatic Choke
VA
915RA115
B
G
VA
915RA116
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Turn the idle speed screw "LA"counterclockwise until thethrottle shaft is clear of the taperon the idle speed screw.
Open the choke shutter "I" andhold it steady in that position.
Swing lever "A" counterclock-wise as far as stop.
Important: The choke shuttermust be held open for the nexttwo operations.
Fit lever "H" on the throttle shaftand rotate it counterclockwiseuntil it butts against the pin onlever "A".
Press lever "H" on to the throttleshaft.
Caution: Take care not to bendthe throttle shaft.
Installed positions of levers:
In the idle position the chokeshutter must be closed and thethrottle shutter at an angle of 37
degrees to the carburetor moun-ting face.
- Open the choke shutter byslowly turning the right-hand endof the choke shaft. The throttleshutter must jump to the closedposition just before the chokeshutter is fully open.
Note: This is accompanied by adefinite clicking sound.
Attach bent end of torsion springto pin on lever "H".
Push the straight end of thetorsion spring between thecarburetor body and pin onlever "A" and locate it behindthe levers pin.
VA
915RA117
L A
VA
915RA119
A
H
VA
915RA118
A
I
VA
915RA120
H
VA
915RA059
37
VA
915RA121
H
VA
915RA122
A
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Checking installed position oftorsion spring:
- Open throttle and choke shutterssimultaneously and hold themsteady.
The torsion spring must move
levers "A" and "B" into thepositions shown in the illustration.
- Remove lever mechanism -see7.15.
- Remove the throttle shaft -see7.12.
Note: The throttle shaft is longer
than on carburetors withoutautomatic choke.
- Remove the choke shutter -see 7.13.
- Ease the E-clip off the chokeshaft.
Pull out the choke shaft.
Remove torsion spring (1),choke lever (2) and torsionspring (3) from the chokeshaft (4).
Install in the reverse sequence.
Slip the grommet, if fitted, off thecarburetor adjusting screws.
Remove screws (1) from intakemanifold, if fitted.
Remove the intake manifold (2)and gasket (3).
- Mark position of intake fittingrelative to fuel pump body.
Take out the screw.
- Remove the end cover.
7.15.1 Throttle Shaft/ 7.16 HL-324A/HL-327D/LA-S8AChoke Shaft Carburetors
B
VA
915RA123A
VA
915RA12
4
VA
915RA125
1
23
4
VA
915RA126
VA
915RA127
3 2
1
VA
915RA128
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Take out the gasket (1) and fuelstrainer (2).
- Clean the fuel strainer.
Take out the screws.
- Remove fuel pump body.
Remove pump diaphragm andgasket.
Note: If diaphragm and gasket arestuck, remove them carefully.
Carefully separate thediaphragm and gasket.
- Inspect diaphragm and gasketand replace if necessary.
Note: The diaphragm and the inletand outlet valves are exposed to
continuous alternating stressesand the material eventually showssigns of fatigue. The diaphragmdistorts and swells and has to bereplaced.
Remove diaphragm cover.
Remove the metering diaphragmand gasket from the carburetorbody or diaphragm cover.
Note: If the gasket and diaphragmare stuck, remove them carefully.
Carefully separate thediaphragm and gasket.
- Inspect diaphragm and gasketand replace if necessary.
Note: The diaphragm is exposedto continuous alternating stressesand the material eventually showssigns of fatigue. The diaphragmdistorts and swells and has to bereplaced.
VA
915RA129
21
VA
915RA132
VA
915RA130
VA
915RA133
VA
915RA134
VA
915RA135
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Take out the screw.
Remove inlet control lever (1)with spindle (2).
Remove the helical spring (3).
Pull out the inlet needle (4).
If there is an annular indentationon the sealing cone of the inletneedle, it will be necessary toreplace the inlet needle becauseit will no longer seal properly.
Use a thin-walled 8mm socketwrench to unscrew the valvebody.
- Take out the gasket.
Unscrew the high speed screw(1), low speed screw (2) and idlespeed screw (3).
Remove washer (1) and 0-ring(2) from the carburetor body orthe adjusting screws.
On HL-327D, unscrew thecontrol valve.
- Take the copper washer outof the bore.
Use a 5 mm (approx. 3/16") pinpunch to press the valve jet outof its seat in the direction of theventuri.
- Wash valve jet in white spirit.
Apply a 3 mm (1/8") pin punch tothe center of the sealing plugand press down until it bucklesand can be taken out.
VA
915RA136
VA
915RA139
VA
915RA137
2
43
1
VA
915RA140
3
1 2
VA
915RA138
1 2
VA
915RA141
VA
915RA142
VA
915RA143
VA
915RA144
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- Wash the carburetor body andall serviceable parts in freshwhite spirit and blow clear withcompressed air, paying specialattention to the bores and ports.
- To replace the carburetor body,
remove the throttle shaft andchoke shaft -see 7.16.1 and 7.16.2.
- Place the new sealing plug in thebore with its convex side facingup.
- Use a 8 mm (5/16") pin punchto apply light pressure until thesealing plug is flat.
- Secure new sealing plug with
Loctite - see 8.2. (Apply Loctiteto gap between carburetor bodyand sealing plug.)
Install the valve jet exactlyvertical in the bore. Press ithome until it is flush with thebottom of the metering chamber.
Fit a new gasket (1).
Fit valve body (2) and tightendown securely.
Insert inlet needle (1).
Fit helical spring (2) in blind hole.
Fit spindle (3) in inlet controllever (4).
- Engage clevis of inlet controllever in annular groove on thehead of the inlet needle.
- Press down the inlet controllever and secure it with the
screw.
Note: Make sure that the helicalspring locates on the controllevers nipple.
- Check easy action of the inletcontrol lever.
Important: The top of the inletcontrol lever must be level with thebottom of the metering chamber.
- If this is not the case, the inletcontrol lever is bent and mustbe replaced.
Fit the gasket and meteringdiaphragm (1).
Note: They are held in position bythe integrally cast pegs (2) on thecarburetor body.
On the LA-S8A, the locatingpegs (1) are on the diaphragmcover. Therefore, the meteringdiaphragm (2) and gasket (3)should be fitted on the
diaphragm cover.
VA
915RA146
2
1
VA
915RA148
VA
915RA149
2
2
2
1
VA
915RA145
1
1
3
1 2
VA
915RA150
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Fit the diaphragm cover.
Fit the gasket (1) and pumpdiaphragm (2).
Note: The gasket and pumpdiaphragm are held in position bythe integrally cast pegs on thediaphragm cover.
Fit the fuel pump body.
- Fit screws and tighten themdown alternately in a diagonalpattern.
- Fit fuel strainer and gasket in thefuel pump body.
Note: Always replace a damagedfuel strainer.
- Fit the end cover.
- Line up the intake fitting accord-ing to mark made prior to dis-assembly.
- Fit screw and tighten downsecurely.
Slip spring (1), washer (2) andnew 0-ring (3) over high speedand low speed screws.
- Carefully screw high speed andlow speed screws down ontotheir seats and then back themoff one full turn.
- Fit idle speed screw.
- On HL-327D, fit new copperwasher in bore for control valve.
- Screw in new control valve about2 full turns.
- Coat the thread that still projectsfrom the carburetor body withLoctite - see 8.2.
- Screw home control valve andtighten down to 4 Nm (3.0 lbf.ft).VA
915RA151
VA
915RA152
2
1
VA
915RA154
123
VA
915RA153
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Take out the screw.
- Remove lever or throttle lever.
Take out the screw.
- Pull the throttle shutter out of thethrottle shaft.
Take out the screw (1).
Pull off the clip (2).
Remove screws (1) from cableguide (2), if fitted.
- Lift away cable guide.
Pull out the throttle shaft.
Remove the torsion spring (1).
After pushing home the throttleshaft, check that torsion springis correctly positioned.
- Install throttle shutter so that itsentire circumference locatesagainst the wall of the venturi.
- Fit screw with Loctite, see 8.2,and tighten down securely.
- Fit clip in the groove.
- Insert screw and tighten downfirmly.
- Install cable guide, if fitted.
On HL-327D, fit lever so that theslotted pin (1) points towardsdiaphragm cover (2).
- Insert screw and tighten downfirmly.
On LA-S8A and HL324A, fitthrottle lever (1) so that roundend (2) point downwards toconnecting flange (3).
- Insert screw and tighten downfirmly.
7.16.1 Throttle Shaft
VA
915RA162
VA
915RA165
1
21
VA
915RA163
VA
915RA166
1
VA
915RA168
2
1
VA
915RA164
12
VA
915RA167
VA
915RA169
2
3
1
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- Remove the intake manifold, iffitted - see 7.16.
Ease the E-clip (1) off thegovernor rod (2), if fitted.
- Remove the governor rod.
On LA-S8A and HL327D, takeout the screw (1).
Remove the governor lever (2).
Take out the screw.
- Pull the choke shutter out of thechoke shaft.
On HL-327D and HL-324A, takeout the screw (1).
Pull off the clip (2).
Withdraw the choke shaft.
Note: On HL-327D, carefully with-draw choke shaft so that the balldoes not pop out and be lost.
- Slide choke shaft into thecarburetor from the adjustingscrew side.
Note: On HL-327D, fit the spring(1) and ball (2).
Push home the choke shaft untilthe ball engages the groove (3).
- Fit clip in groove in choke shaft.
- Secure clip with screw.
On LA-S8A and HL327D, rotatechoke shaft so that slot (1) ishorizontal and the flat side (2)points down.
7.16.2 Choke Shaft
VA
915RA155
2
1
VA
915RA158
2
1
VA
915RA16
0
2
1
3
VA
915RA156
12
VA
915RA159
VA
915RA157
VA
915RA161
2 1
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On HL-324A, rotate choke shaftso that the pin (1) points forward.
Fit the choke shutter so that thehole (2) points forward and theindentations (3) face the endcover.
- Fit screw with Loctite, see 8.2,and tighten down securely.
- On LA-S8A and HL327D, closethe choke shutter.
Fit governor lever (1) so that thepin (2) lines up with the center ofthe high speed screw (3).
- Fit screw and tighten down
securely.
Take out the screws (1).
Remove the end cover (2) withcap.
Pull out the cap.
Remove the flange.
Take valve out of flange.
- Check valve and replace ifnecessary.
Note: If the circuit for filling themetering chamber is malfunction-
ing, if the valve is dirty, distortedor damaged, install a new valve.
Remove metering diaphragmand gasket.
Note: If the gasket and diaphragmare stuck, remove them carefully.
VA
915RA055
2 1
3
VA
915RA192
2
1 1
11
VA
915RA191
VA
915RA193
VA
915RA171
3
12
VA
915RA194
VA
915RA196
7.17 WYL-63 / WYL-73Rotary Valve Carburetors
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Carefully separate thediaphragm and gasket.
- Inspect diaphragm and replaceif necessary.
Note: The diaphragm is exposedto continuous alternating stresses
and the material eventually showssigns of fatigue. The diaphragmdistorts and swells and has to bereplaced.
Remove pump housing.
Remove the screw (1).
Remove inlet control lever (2)with spindle (3) and spring.
- Pull out the inlet needle.
If there is an annular indentationon the sealing cone of the inletneedle, it will be necessary toreplace the inlet needle becauseit will no longer seal properly.
Use a scriber to pry out fuelstrainer and then clean it.
Important: Always replace adamaged fuel strainer.
Remove gasket and pumpdiaphragm.
Note: If gasket and diaphragm arestuck, remove them carefully.
Carefully separate thediaphragm and gasket.
- Inspect diaphragm and replace ifnecessary.
Note: The diaphragm is exposedto continuous alternating stressesand the material eventually showssigns of fatigue. The diaphragmdistorts and swells and has to bereplaced.
VA
915RA195
VA
915R
A197
1
23
VA
915RA201
VA
915RA199
VA
915RA20
2
VA
915RA198
VA
915R
A200
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Remove the spring.
Use a scriber to carefully easethe main jet out of its seat.
- Remove the sealing ring.
Take out the screws (1).
Pull out the control valve (2).
If necessary, pry off the retainingring (1) and pull out the slottedpin (2) with washer (3).
Take out the idle speedscrew (4).
- Wash the carburetor body and
all serviceable parts in freshwhite spirit and blow clear with
compressed air, paying specialattention to the bores and ports.
Install the control valve so thatthe idle speed screw (1) points tothe left - viewed from stub (2).
- Insert screws and tighten downsecurely.
Slip 0-ring (1) over the mainjet (2).
- Press main jet into carburetorseat as far as stop.
Note: Check size of main jet(number on main jet):40 = WYL-6336 = WYL-73
VA
915RA203
VA
915RA206
3
4
2
1
VA
915RA204
40
VA
915RA205
1
1
2
VA
915RA207
2
1
VA
915RA208
1
2
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Fit the spring (1).
Fit a new gasket (2).
Place pump diaphragm on thegasket.
Note: Gasket and pumpdiaphragm are held in placeby the integrally cast pegs.
Fit the inlet needle (1).
Fit helical spring (2) in the blindhole.
Insert spindle (3) in the inletcontrol lever (4).
- Engage clevis of inlet controllever in annular groove on thehead of the inlet needle.
- Press down the inlet controllever and secure it with thescrew.
Note: Make sure that the helicalspring locates on the controllevers nipple.
Important: Measure distance "a"between upper edge of inlet con-trol lever and pump housing. Itmust be 1.5 mm (0.06").
- If the distance more or less thanspecified, the inlet control leveris bent and must be replaced.
Fit the pump housing.
Fit a new gasket.
Fit diaphragm on the gasketso that perforated plate pointstowards the inlet control.
2
VA
915RA209
1
VA
915RA21
3
VA
915RA210
a
915RA212VA
VA
915RA21
4
VA
915RA21
1
1
2
3
4
VA
915RA21
5
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Note: Gasket und pumpdiaphragm are held in placeby the integrally cast pegs.
- Insert the valve in the flange.
Fit the flange in position.
- Fit the cap in the end cover.
Fit the end cover.
- Insert screws and tighten downsecurely.
Adjusting idle speed andidle mixture
Note: If either the carburetor orthe control valve has been re-placed, it will be necessary toadjust idle jet needle in the controlvalve.
Set the idle speed to 3100
200 rpm with the idle speedscrew (1).
Note: Use a tachometer.
Pull plug (2) out of the controlvalve.
- Start the engine.
Use a small screwdriver (3) torotate the idle jet needle (4)
clockwise or counterclockwiseuntil maximum engine speed isreached.
- From that position, turn the idlejet needle one eight turn coun-ter clockwise to reduce enginespeed by 200 - 500 rpm.
- Remove the screwdriver andonce more set idle speed to
3100 200 rpm with the idlespeed screw.
Note: Adjustment of maximumengine speed is not necessarybecause it is determined by themain jet.
- Fit plug in the control valve.
VA
915RA194
VA
915RA
216
VA
915RA217
1
4
2 3
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WT-283B, WT-323, WT-325A,WT-326C, WT-360, WT-396,WT-403A, WT-493, WT-498A,WT-503
These carburetors have a fixed jetin place of a high speed adjusting
screw (H screw).
Maximum engine speed is presetand no longer adjustable. It is onlypossible to correct idling speedwithin certain limits.
These carburetors guarantee anoptimum fuel-air mixture in alloperating conditions.
Standard setting
To readjust the carburetor, startwith the standard setting.
Carefully screw down the lowspeed screw (L) clockwise until itis against its seat. Then back itoff one full turn counterclockwise.
- Check chain tension.
- Check air filter and clean ifnecessary.
- Check spark arresting screenand clean or replace if necessary.
- Start the engine and allow it towarm up.
Adjusting idle speed
A correction at the low speedscrew (L) usually necessitates achange in the setting of the idle
speed screw (LA).
Note: Turn screws very slowly andcarefully - even slight movementsproduce a noticeable change inengine running behavior.
Engine stops while idling:
- Check standard setting.
- Turn idle speed screw clockwiseuntil the chain begins to run -then turn it back one quarter turn.
Chain runs while engine isidling:
- Check standard setting.
- Turn the idle speed screwcounterclockwise until the chainstops running - and then turn itabout another quarter turn inthe same direction.
Erratic idling behavior, pooracceleration -even though low speed screw (L)is one turn open:
- Idle setting is too lean.
- Turn the low speed screw (L)counterclockwise until the engineruns and accelerates smoothly.
VA
915RA294
L
VA
915RA295
LA
7.18 Standard Setting on Walbro Carburetors7.18.1 Fixed Jet Carburetors
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HD-13A, HD-16, HD-17,HD-18A, HD-21A, HD-22,WY-69, WT-329, WT-330,WT-367, WT-393, WT-489,WT-492, WJ-69
Turn the cap to line up the lugwith the slot.
Screw puller (1) 5910 890 4500into cap counterclockwise untilit comes off the the head of theadjusting screw (left-handthread).
Note: Limiter caps are damagedduring removal. They must not beused again.
- If necessary, take out the lowspeed and high speed screws.
Carefully screw high speedscrew (1) and low speed screw(2) with sealing rings (3) downonto their seats.
- Refer to flow chart for carburetoradjustment - see 7.21.
Preparations
- Check air filter and clean ifnecessary.
- Inspect spark arresting screenand clean or replace asnecessary.
On blowers:
- Fit blower tubes and nozzles.
On hedge trimmers:
- Clean and oil cutting blades.
One brushcutters:
- Mount STIHL Autocut orPolymatic cutting head.
- Adjust line length so that itextends as far as limiter blade
on deflector.
Basic setting(all models)
Starting with the adjustingscrews (H) and (L) firmly againsttheir seats, open them by thenumber of turns specified below.
Model H L
BR 320 LBR 320, 400FC 44FS 36, 40, 44
FS 72, 74, 76FS 88FS 550HS 72, 74, 76BG 72MS 044, 046MS 029, 039MS 066
1.5241.5
1
111
42.5
4
1
111
7.18.2 Carburetors with Limiter Caps
VA
915RA218
1
VA
915RA219
2 3
1
VA
915RA225
VA
915RA293
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MS 029/039 and MS 044/046
Fitting limiter caps
Insert the new caps in the bores,making sure the lugs arecorrectly positioned (arrows).
Push the caps on to the adjust-ing screws until they are flushwith the insert (1).
Adjusting idle speed
Note: Use a tachometer.
Set idle speed to 2800 rpm(029/039) or 2500 rpm (044/046)with the idle speed screw (2).
Corrections are now possibleonly within a range of270 degrees because theintegrally molded stops in thebores limit any further adjust-
ment.
- This enables the mixture to bemade leaner, if necessary, foroperation at high altitudes andprevents it being enrichedbeyond the emission limitsspecified by EPA.
BR 320 L, BR 320, 400,FC 44, FS 36, 40, 44,FS 72, 74, 76, FS 88, FS 550,HS 72, 74, 76, BG 72
Adjusting idle speed
- Refer to flow chart for carburetoradjustment - see 7.21.
Use the low speed screw (L) toset maximum idle speed.
Turn low speed screw (L)counterclockwise until enginespeed drops about 300 rpm.
Use the idle speed screw (LA)
to set engine speed as specifiedbelow.
Model RPM
BR 320 LBR 320, 400FC 44FS 36, 40, 44FS 72, 74, 76FS 88FS 550HS 72, 74, 76BG 72
31003100310031002800
Full-load setting
On blowers:
Running at full load, turn highspeed screw (H) slowly clock-wise until maximum enginespeed is reached.
Turn high speed screw (H)counterclockwise until enginespeed drops about 100 rpm.
VA
915RA229
VA
915RA22
7
270 270
H L
VA
915RA228
52 Carburetors
-
7/30/2019 Stihl Carburetor Manual
53/65
On brushcutters(with cutting head and deflectorfitted):
Running at full load, turn highspeed screw (H) slowly clock-wise until no further increasein engine speed is achieved
within a speed range of 6500 -8500 rpm.
Note: If the required speed rangeis not reached, adjust the length ofthe nylon line until it is.
Turn high speed screw (H)counterclockwise until enginespeed drops about 100 rpm.
On edgers:
Running at full load, turn highspeed screw (H) slowly clock-wise until engine speed of9000 rpm is exceeded.
Turn high speed screw (H)counterclockwise until an enginespeed of about 8000 rpm isreached.
Turn high speed screw (H) onehalf turn clockwise (leaner).
Checking running behavior
- Open the throttle several timesto check that engine acceleratessmoothly.
- Engine must run uniformly at
idle speed.
Fitting limiter caps
Fit cap (1) on adjusting screw sothat the integrally molded lug (2)locates in the gate (3) in the bore.
- Push caps into bores until theyare flush with the carburetorbody.
Corrections are now possibleonly within a range of 90degrees because the integrallymolded stops in the bores limitany further adj