Carburetor TheoryCarburetor Theory

Paul Crovella Paul Crovella

AGEN 220AGEN 220

Variable Venturi or “slide” Variable Venturi or “slide”

carburetorscarburetors

The goal of providing The goal of providing the stochiometric ratio, the stochiometric ratio, (14.7:1) is tempered with (14.7:1) is tempered with the goal of providing the goal of providing extra fuel to cool (by extra fuel to cool (by evaporation) the engine.evaporation) the engine.

Carburetor tuning must Carburetor tuning must be done to provide max be done to provide max power without power without overheating the 2 stroke overheating the 2 stroke engine.engine.

VM CarburetorVM Carburetor

Slide carburetor provides Slide carburetor provides smoother variation and smoother variation and better adjustment than a better adjustment than a throttle plate carburetor.throttle plate carburetor.

VM is a Mikuni round VM is a Mikuni round slide carburetor.slide carburetor.

Various circuits control Various circuits control fuel and air flow for fuel and air flow for different conditionsdifferent conditions

Starting, (idle) pilot, and Starting, (idle) pilot, and main main

Integrated carburetor systems Integrated carburetor systems

The carburetor has The carburetor has multiple adjustments for multiple adjustments for different throttle different throttle positions;positions;

00--1/4 Pilot jet1/4 Pilot jet

1/81/8--1/2 Throttle cutaway1/2 Throttle cutaway

¼¼--3/4 Jet needle (clip)3/4 Jet needle (clip)

¼¼--3/4 Needle jet3/4 Needle jet

½½--Full Main jetFull Main jet

½½--Full Air adjustment Full Air adjustment screwscrew

½½--full Power jet (optional)full Power jet (optional)

Starter CircuitStarter Circuit

MiniMini-- slide carburetor slide carburetor used for startingused for starting

“Choking” the “Choking” the carburetor actually opens carburetor actually opens plunger to allow gas flowplunger to allow gas flow

Gas is drawn from the Gas is drawn from the bottom of the bowl bottom of the bowl through a starter jetthrough a starter jet

Air is drawn from in Air is drawn from in front of the slidefront of the slide

Starter circuitStarter circuit

Starter CircuitStarter Circuit

Starter can only function when slide is closed Starter can only function when slide is closed

and vacuum is highand vacuum is high

Starter jet may need to be changed if using a Starter jet may need to be changed if using a

snowmobile carb on a motorcycle, or vicesnowmobile carb on a motorcycle, or vice--versaversa

Plunger positions Plunger positions –– Open (choked), halfOpen (choked), half--open, open,

closedclosed

Starter circuit can be used to enrichen mixture at Starter circuit can be used to enrichen mixture at

other times to check carburetor problems.other times to check carburetor problems.

Pilot circuitPilot circuit

Used at idle to ¼ throttleUsed at idle to ¼ throttle

Gas flow is controlled by Gas flow is controlled by pilot jet size, this jet is the pilot jet size, this jet is the first to plug if gas thickensfirst to plug if gas thickens

Air flow is controlled by air Air flow is controlled by air adjustment needleadjustment needle

At idle, air flows in at 5 to At idle, air flows in at 5 to further mix with fuelfurther mix with fuel

At just off idle, mixed fuelAt just off idle, mixed fuel--air flows out at both 6 and air flows out at both 6 and 5 5

Pilot CircuitPilot Circuit

Stumble coming off idle Stumble coming off idle indicates plugged pilot jet indicates plugged pilot jet or air screw too tightor air screw too tight

Idle speed controlled by Idle speed controlled by a separate screw that a separate screw that determines how tightly determines how tightly the slide closes.the slide closes.

Slides with different Slides with different shape cutouts are also shape cutouts are also sold for tuning sold for tuning applications.applications.

Mid range operationMid range operation

¼ to ¾ operation is ¼ to ¾ operation is controlled by needle jet controlled by needle jet and jet needle. As slide and jet needle. As slide pulls up the long tapered pulls up the long tapered needle is drawn out of needle is drawn out of needle jet.needle jet.

The air jet controls the The air jet controls the air bled in to atomize the air bled in to atomize the fuel.fuel.

The main jet is so large The main jet is so large that it does not limit flowthat it does not limit flow

Mid range operationMid range operation

Both needle jet and jet Both needle jet and jet

needles can be changed, needles can be changed,

as can the position of the as can the position of the

clip on the end of the clip on the end of the

needle.needle.

Needles can have up to Needles can have up to

four different tapers on four different tapers on

them for different them for different

performance throughout performance throughout

the slide travel.the slide travel.

TuningTuning

Simple tuning can begin Simple tuning can begin

with moving the needle with moving the needle

up or down one clip up or down one clip

position. This can be position. This can be

done before changing done before changing

needles or needle jets.needles or needle jets.

Any tuning adjustments Any tuning adjustments

should be made by should be made by

stepping jets up or down stepping jets up or down

oneone size at a time.size at a time.

Full throttle operationFull throttle operation

At ¾ to full throttle, the At ¾ to full throttle, the

needle is so far removed needle is so far removed

from the needle jet, that from the needle jet, that

it no longer limits flow. it no longer limits flow.

At this throttle range, the At this throttle range, the

main jet limits gas flow.main jet limits gas flow.

Air jet limits air flow.Air jet limits air flow.

Always jet main rich and Always jet main rich and

work down.work down.

Air jet is typically not Air jet is typically not

changedchanged

Power jetsPower jets

Not found on all carbs Not found on all carbs –– Provides richer mix at top endProvides richer mix at top end

Jetting adjustmentsJetting adjustments

To make proper To make proper

carburetor adjustments carburetor adjustments

the mixture ratio at each the mixture ratio at each

range will have to be range will have to be

determined. There are determined. There are

various methods:various methods:

Plug reading Plug reading –– plug chopplug chop

Piston washPiston wash

Pyrometer readingsPyrometer readings

Condition correctionsCondition corrections

Exhaust soundExhaust sound

Plug Reading Plug Reading –– Plug ChopPlug Chop

To get an accurate read of To get an accurate read of the condition, perform a the condition, perform a “plug chop”“plug chop”

Install a new plugInstall a new plug

Warm engine upWarm engine up

Run engine at selected throttle Run engine at selected throttle setting for 30 seconds setting for 30 seconds –– 1 1 minute.minute.

Cut ignition and Cut ignition and simultaneously closesimultaneously close--off carboff carb

Study insulator down toward Study insulator down toward the base of the insulatorthe base of the insulator, not , not on the tip!on the tip!

Appearance of tip better for Appearance of tip better for determining detonationdetermining detonation

Plug appearancePlug appearance

Center electrodesCenter electrodes

Another valuable indicator on the spark plug is the center firing electrode; as the plug color starts to lighten up when properly jetted, the center firing electrode will start to have a “silver” tip, or crown. As the mixture becomes leaner, this silver crown will start to creep down the side of the electrode; this is your target. This metallic appearance on the end of the electrode should not extend any further than 1/4 - 1/3 of the way down the tip. Many tuners are happy with the margin afforded by simply seeing the silver tip; then they’re close enough for trail riding with a bit of margin to spare.

Side electrodeSide electrode

The ground electrode is also an indicator; on many engines you will see a “shadow” (darker area) just up to the radius (bend) that will usually coincide with the color and firing electrode appearance. If this shadow is further down the ground strap towards the plug threads, you’re likely too lean. If this dark shadow is all the way across the strap to the center of the plug, you’re too rich.

NGK plug numbering systemNGK plug numbering system

The plug heat range can be The plug heat range can be adjusted to keep a plug tip clean, or adjusted to keep a plug tip clean, or avoid detonating on an engine that avoid detonating on an engine that otherwise has the right mixture. otherwise has the right mixture.

For NGK and most foreign For NGK and most foreign manufacturer’s spark plugs there is manufacturer’s spark plugs there is a simple rule of thumb:Low heat a simple rule of thumb:Low heat rating number (for inst. BP4ES) rating number (for inst. BP4ES) "Hot spark plug".high heat intake "Hot spark plug".high heat intake due to long insulator tip.due to long insulator tip.

.High heat rating number (for inst. .High heat rating number (for inst. BP8ES) "Cold spark plug".Low BP8ES) "Cold spark plug".Low heat intake, due to short insulator heat intake, due to short insulator tip.tip.

For Champion and most domestic For Champion and most domestic manufacturer’s spark plugs (e.g. in manufacturer’s spark plugs (e.g. in ATVs, the numbering is opposite.ATVs, the numbering is opposite.

Piston readingPiston reading

Normally done for an engine that has been run for ¼ Normally done for an engine that has been run for ¼ mile at constant speed. By looking down the spark plug mile at constant speed. By looking down the spark plug hole with piston at BDC, a clear look at the top of the hole with piston at BDC, a clear look at the top of the piston can determine how much carbon is left. A ½” piston can determine how much carbon is left. A ½” ring of wash (clean) is “normal”, typically a little ring of wash (clean) is “normal”, typically a little variation at transfer and exhaust ports will be noted.variation at transfer and exhaust ports will be noted.

Exhaust gas pyrometersExhaust gas pyrometers

Uses the Exhaust Uses the Exhaust

temperature as a check on temperature as a check on

mixture, and relative changes, mixture, and relative changes,

not for baseline tuning. not for baseline tuning.

Thermocouples are typically Thermocouples are typically

installed permanently 5installed permanently 5--10” 10”

from exhaust port and they from exhaust port and they

must be centered in pipe. must be centered in pipe.

Manufacturers will often give Manufacturers will often give

you an exact location.you an exact location.

Interpreting EGT valuesInterpreting EGT values

Each gauge is used for Each gauge is used for

relative readingsrelative readings

Guidelines Guidelines

Below 1100 Below 1100 –– RichRich

12001200--IdealIdeal

Above 1250 LeanAbove 1250 Lean

Changing a Mikuni Changing a Mikuni

carburetor jet one size will carburetor jet one size will

cause about a 50 degree cause about a 50 degree

temperature changetemperature change

Relative Air Density correctionRelative Air Density correction

Once an ideal jetting has Once an ideal jetting has

been developed for one been developed for one

condition, it will need to be condition, it will need to be

corrected for changing corrected for changing

environmental conditions:environmental conditions:

Temperature (Loss of 2% per Temperature (Loss of 2% per

10 degree F increase)10 degree F increase)

Altitude (loss of 3.5% per Altitude (loss of 3.5% per

1000’ increase)1000’ increase)

Barometric pressureBarometric pressure

Relative Humidity Relative Humidity –– Moist air Moist air

is less denseis less dense

Relative Air density calculationsRelative Air density calculations

Calculating Air DensityCalculating Air Density

Air density is expressed in units of mass per unit volume, in this case kg/m3. The Air density is expressed in units of mass per unit volume, in this case kg/m3. The formula for this calculation is derived from basic physics.formula for this calculation is derived from basic physics.

Air Density (kg/m3) = 1.2929 * (273.13/(T+273.13)) * ((PAir Density (kg/m3) = 1.2929 * (273.13/(T+273.13)) * ((P--MN*RH)/760)MN*RH)/760)

Simplified = 0.46464*(PSimplified = 0.46464*(P--MN*RH)/(T+273.13)MN*RH)/(T+273.13)

where where

T = temperature in Celsius T = temperature in Celsius

P = barometric pressure in mm of mercury P = barometric pressure in mm of mercury

MN = moisture number from the following chart MN = moisture number from the following chart -- include chartinclude chart

RH = relative humidity, expressed as a decimal (0.55, not 55%) RH = relative humidity, expressed as a decimal (0.55, not 55%)

For example: For example:

T = 13C T = 13C

P = 770 mmHg P = 770 mmHg

MN per the chart for 13C is 11.24 MN per the chart for 13C is 11.24

RH = .21 (21%)RH = .21 (21%)

Air Density = 1.25 kg/m3 Air Density = 1.25 kg/m3

Relative air density useRelative air density use

In simple form, the change in relative air density In simple form, the change in relative air density

corresponds to the change in jet size.corresponds to the change in jet size.

e.g. e.g. –– If relative air density goes from 100 to 103 (a 3 % If relative air density goes from 100 to 103 (a 3 %

change) the jet size should increase by 3%. If you are using a change) the jet size should increase by 3%. If you are using a

300 jet, the new jet size will be (300 x 1.03) = 309. Since jets 300 jet, the new jet size will be (300 x 1.03) = 309. Since jets

only come in multiples of 5, you could use a 310. only come in multiples of 5, you could use a 310.

Jetting changes in direct proportion to changes in absolute temp on Jetting changes in direct proportion to changes in absolute temp on

the Rankine scale (460 + Farenheit temp)the Rankine scale (460 + Farenheit temp)

Jetting changes only 70% as much as absolute barometric pressure Jetting changes only 70% as much as absolute barometric pressure

changes. This is due to the fact that the bowl is vented, and less changes. This is due to the fact that the bowl is vented, and less

pressure is exerted on the gas in the bowl as wellpressure is exerted on the gas in the bowl as well

Hex head jets are marked in area, round head jets in diameter.Hex head jets are marked in area, round head jets in diameter.

Every change of 3 on a RAD gauge, changes the main jet one sizeEvery change of 3 on a RAD gauge, changes the main jet one size

Correcting jet size for RADCorrecting jet size for RAD

For instance, if you get the jetting perfect at AD = 1.05 and the For instance, if you get the jetting perfect at AD = 1.05 and the air density changes to 1.08, you know that you will need to go up air density changes to 1.08, you know that you will need to go up at least one size. If it goes from 1.05 to 1.06, is it time to change at least one size. If it goes from 1.05 to 1.06, is it time to change a jet or not? This is why you need to check all the way through a jet or not? This is why you need to check all the way through the range. I know that for my bike, AD = 1.03the range. I know that for my bike, AD = 1.03--1.05 is one jet 1.05 is one jet size and 1.06size and 1.06--1.08 is the next size up. In the above example, I 1.08 is the next size up. In the above example, I would have known to go up one size. If it had gone up to 1.08 would have known to go up one size. If it had gone up to 1.08 and looked like it was going to continue rising, I would go up and looked like it was going to continue rising, I would go up two sizes.two sizes.

Engine soundEngine sound

An engine running way An engine running way

too rich will “four cycle” too rich will “four cycle”

or miss ignition on every or miss ignition on every

other stroke.other stroke.

An engine running way An engine running way

too lean will be running too lean will be running

very fast and not return very fast and not return

to idleto idle

TM CarburetorTM Carburetor

TM is a flat slide carbTM is a flat slide carb

Less disruption of air Less disruption of air flow in venturi gives flow in venturi gives carbs better response and carbs better response and power.power.

Rack mounted for twins Rack mounted for twins and triples reduce and triples reduce throttle effort, and are throttle effort, and are held in synchronizationheld in synchronization

Circuits similar to VMCircuits similar to VM

Mikuni CV CarbMikuni CV Carb

Used on four strokes, Used on four strokes, this is a “Constant this is a “Constant Velocity” or “Constant Velocity” or “Constant Vacuum” carb.Vacuum” carb.

Butterfly throttle is Butterfly throttle is combined with a slide to combined with a slide to give the carb good give the carb good response to quick response to quick throttle opening.throttle opening.

Butterfly controlled by Butterfly controlled by operator, Slide is operator, Slide is controlled by vacuumcontrolled by vacuum

CV Carb OperationCV Carb Operation

Operator controls butterfly Operator controls butterfly from idle to ¼ throttle, from idle to ¼ throttle, during this slide is held ¼ during this slide is held ¼ openopen

Past ¼ throttle, the engine Past ¼ throttle, the engine vacuum is applied to the vacuum is applied to the slide, and the slide movement slide, and the slide movement is controlled by vacuumis controlled by vacuum

Slide will not open until Slide will not open until sufficient engine vacuum has sufficient engine vacuum has been developed.been developed.

Opening throttle quickly will Opening throttle quickly will not produce “flat spot”not produce “flat spot”

Setting Idle speed too high Setting Idle speed too high will cause carburetor to will cause carburetor to “hang”, or not return to idle“hang”, or not return to idle

Carburetor sizeCarburetor size

Carburetor size must be Carburetor size must be

matched to flow for matched to flow for

displacement and rpms displacement and rpms

that engine is used. that engine is used.

Changing either will Changing either will

require an adjustment to require an adjustment to

the carburetor to match.the carburetor to match.

Carburetor Throttle Carburetor Throttle

Bore DiameterBore Diameter

D = K x SQRT( C x N ) D = K x SQRT( C x N )

D is throttle bore D is throttle bore

diameter, in millimeters diameter, in millimeters

K is a constant ( approx. K is a constant ( approx.

0.65 to 0.9, derive from 0.65 to 0.9, derive from

existing carburetor bore) existing carburetor bore)

C is cylinder C is cylinder

displacement, in liters displacement, in liters

N is RPM at peak power N is RPM at peak power

Tuned intakeTuned intake

Intake (airbox) can also Intake (airbox) can also

be “tuned” to match be “tuned” to match

resonance of intake with resonance of intake with

peak power band.peak power band.

Resonance EffectsResonance Effects

F = Vs / 2¼ * the square F = Vs / 2¼ * the square

root of A / Vc (L + 1/2 the root of A / Vc (L + 1/2 the

square root of square root of ¼ A ¼ A

Vs is the sonic speed Vs is the sonic speed

Usually about 1100 ft/sec) Usually about 1100 ft/sec)

A is the crossA is the cross--sectional area sectional area

of the inlet of the inlet

L is the inlet pipe length L is the inlet pipe length

Vc is the flask (crankcase) Vc is the flask (crankcase)

volume volume

Ignition safetyIgnition safety

To avoid having the slides To avoid having the slides freeze open and customer freeze open and customer stuck on a runaway sled, the stuck on a runaway sled, the safety switch on the side of safety switch on the side of carb measures a magnet on carb measures a magnet on the slide to be sure the slide the slide to be sure the slide closes when throttle is closes when throttle is released. released.

When replacing throttle When replacing throttle cable, BE SURE TO SET cable, BE SURE TO SET FREE PLAY or engine may FREE PLAY or engine may not startnot start