Carburetion

Purpose of carburetor

To provide proper ratio of air and fuel under every condition for good performance, economy and emissions

Fuel must be atomized to be vaporized

Venturi principal

Air flowing through a tube will cause a pressure drop in that tube

Adding a restriction or two to a tube will cause a larger decrease in pressure (increase in vacuum)

This vacuum is used to suck or draw fuel from the float bowl

Multiple barrel carbs.

Primary vs. secondary barrels– Progressive vs. non-progressive

One - two - three - four

Carburetors have 2 or 3 main parts

Air horn, float bowl, throttle body or air horn, main body

Carb overhaul

Chemicals– Aerosol carb cleaner– Carburetor (cold parts) cleaner dip

Soak & rinse Blow out Replace all gaskets Perform all adjustments

Control of fuel to air ratio is primarily controlled by seven

different circuits

Circuits

Float circuit Idle circuit Transfer (off idle) circuit Main circuit Power circuit Accelerator pump circuit Choke circuit

Float circuit : kind of fuel tank

Purpose: to store fuel for all other systems to work properly– If float system is malfunctioning, all other

systems will be affected

Float level adjustment

Critical to all other circuits Must be checked on overhaul

Problems with float circuit

High float levels– Can be caused by sunk float or float out of

adjustment– Will cause rich condition

Needle and seat sticking / debris on seat– Usually causes flooding over– Sometimes can be flushed out

Problems with float circuit

Low float levels– Caused by improper adjustments– Will cause lean condition

Idle circuit / slow circuit

Purpose: to supply engine with fuel at speeds too slow to operate main circuit

Idle circuit components and operation

Starts through main jet Idle restrictor / slow jet Air bleeds Fuel cut / slow cut solenoid

– Solenoid will have the ability to shut off all fuel in idle circuit

Idle circuit components and operation

Idle mixture screw– Usually out to richen / in to lean– Do not bottom– Spring on screw is to help maintain

adjustment

Idle problems

Plugged passages Failing solenoids and/or circuits Plugged air bleeds

Idle speed adjusment

Usually adjustments are accomplished by turning appropriate screw

Idle stop solenoids – Electric or vacuum

Curb idle means no load at stop

Idle mixture

Perform only after checking for vacuum leaks

Screw anti-tamper devises If two / four barrel keep both screws the

same Adjust to factory recommendations

Transfer / off idle circuit

Purpose: supplies fuel at speeds too slow for main circuit

Limited by idle restrictor Eliminates one idle air bleed but adds

volume to idle circuit Used on light acceleration

Transfer circuit problems

Plugging causing a lean hesitation More noticeable on smaller

displacement engines Usually older people give complaints

Main circuit / main metering

Purpose; to supply fuel to engine at cruise speeds

Components and operation

Jet (stamped numbers on jet) Atomizer / emulsion tube Air bleed

– Altitude compensators– At nozzle stops dripping, possibly due to

high idle speed Discharge nozzle Venturi / boost venturis

Problems and adjustments

Plugging jets and air bleeds Jets change / do not drill (causes

swirling) Rod changing Float level increases

Power circuit

Purpose: to enrichen mixture through main circuit for increased power under load

Components and operation

Most work on manifold vacuum– Manifold vacuum low at W.O.T.

Some have jet– Calibrated size with stamp number– May incorporate spring and valve

Diaphragm type (Holleys) Power piston on jet Power piston with rods Power piston return springs

Problems and adjustments

Vacuum piston stick on or off– Rod type – Valve type

Diaphragm rupturing– Possibly caused by backfiring– Often can be accessed externally

Accelerator pump circuit

Purpose: to supply engine with shot of fuel on moderate - heavy acceleration

Necessary because fuel is heavier than air

Components and operation

Plunger in machined bore (hooked to linkage)

Diaphragm (hooked to linkage by cam) Inlet and outlet check balls, weights and

springs Pump discharge jet Duration and return springs

Inspection of system

Look for solid stream No external leaks

Problems and adjustments

Plunger– Cup material deteriorating

Diaphragm– Leaking externally due to oxygenated fuels

Problems and adjustments

Check balls leaking Outlet ball sticking Adjustments by hole in arms Adjustments by cams

Choke circuit

Purpose: to enrichen fuel mixture on cold conditions and to raise idle speeds cold

Fuel vaporizes slower cold causing much of the fuel to be wasted

Vehicle does not run as efficiently cold needing more idle speed

Components and operation

Plate or butterfly– Draws fuel from all points when closed

including air bleeds– Increases venturi vacuum under plate

when closed

Components and operation

Pulloff or brake– Purpose: to open choke plate a desired

amount at first start to obtain a proper a/f ratio– Manifold vacuum controlled

Vacuum can be computer controlled Vacuum can be tvs controlled

– Can have a primary and secondary for two stages

– May have a delay valve

Components and operation

Coil– Electric– Heated air– Divorced– Water

Fast idle cam

Adjustments

Choke coil– Alignment of index marks

Choke pulloff– Drill diameter– Angle gauge– Calibrated vacuum leaks

Fast idle speed

Problems

Choke coil not warming up Plate sticking Choke pulloff rupturing Fast idle speed adj.

Secondary(s)

Purpose: to increase volume of air fuel mixture under load

Components and operation

Usually has only a main circuit May also have idle, and accelerator pump

circuits Can be mechanical or vacuum controlled Can have air valves controlled by choke

pulloffs Usually have a choke lock out