Carburetion

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Carburetion
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Purpose of carburetor

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

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Fuel must be atomized to be vaporized
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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

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Multiple barrel carbs.

• Primary vs. secondary barrels
• Progressive vs. non-progressive
• One - two - three - four

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Carburetors have 2 or 3 main parts

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

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Carb overhaul

• Chemicals
• Aerosol carb cleaner
• Carburetor (cold parts) cleaner dip
• Soak rinse
• Blow out
• Replace all gaskets
• Perform all adjustments

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Control of fuel to air ratio is primarily
controlled by seven different circuits
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Circuits

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

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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

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Float level adjustment

• Critical to all other circuits
• Must be checked on overhaul

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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

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Problems with float circuit

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

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Idle circuit / slow circuit

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

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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

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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

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Idle problems

• Plugged passages
• Failing solenoids and/or circuits
• Plugged air bleeds

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Idle speed adjusment

• Usually adjustments are accomplished by turning
  appropriate screw
• Idle stop solenoids
• Electric or vacuum
• Curb idle means no load at stop

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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

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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

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Transfer circuit problems

• Plugging causing a lean hesitation
• More noticeable on smaller displacement engines
• Usually older people give complaints

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Main circuit / main metering

• Purpose to supply fuel to engine at cruise speeds

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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

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Problems and adjustments

• Plugging jets and air bleeds
• Jets change / do not drill (causes swirling)
• Rod changing
• Float level increases

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Power circuit

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

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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

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Problems and adjustments

• Vacuum piston stick on or off
• Rod type
• Valve type
• Diaphragm rupturing
• Possibly caused by backfiring
• Often can be accessed externally

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Accelerator pump circuit

• Purpose to supply engine with shot of fuel on
  moderate - heavy acceleration
• Necessary because fuel is heavier than air

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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

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Inspection of system

• Look for solid stream
• No external leaks

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Problems and adjustments

• Plunger
• Cup material deteriorating
• Diaphragm
• Leaking externally due to oxygenated fuels

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Problems and adjustments

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

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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

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Components and operation

• Plate or butterfly
• Draws fuel from all points when closed including
  air bleeds
• Increases venturi vacuum under plate when closed

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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

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Components and operation

• Coil
• Electric
• Heated air
• Divorced
• Water
• Fast idle cam

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Adjustments

• Choke coil
• Alignment of index marks
• Choke pulloff
• Drill diameter
• Angle gauge
• Calibrated vacuum leaks
• Fast idle speed

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Problems

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

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Secondary(s)

• Purpose to increase volume of air fuel mixture
  under load

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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