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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Throttle plate is used to control amount of air /
fuel entering the engine

• Limits volumetric efficiency
• Limits venturi vacuum

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Multiple barrel carbs.

• Primary vs. secondary barrels
• One - two - three - four

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CFM cubic feet per minute

• What is a cubic foot?

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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
• 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) curcuit
• 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
• Components and operation
• Float
• Needle seat
• Hinge
• Bowl vents internal and external
• Bowl displacers
• Dampening springs
• Float types
• Nitrile (looks like a sponge)
• Hollow brass
• Hollow plastic
• Adjustments
• Level
• Bending arm while using float gauges (with or
  without gasket)
• Adjusting seat externally adjustable (holleys)
• Sight glass with dot in the middle
• Drop adjustment eliminating float binding
• Problems with circuit

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Idle circuit / slow circuit

• Purpose to supply engine with fuel at speeds too
  slow to operate main circuit
• Starts through main jet (brief explanation)
• To idle restrictor / slow jet
• Fuel cut / slow cut solenoid
• Solenoid operation
• Air bleeds
• Start atomization
• Air bleed jets
• Altitude compensators
• Transfer ports / work as air bleeds
• Idle mixture screw
• Out to richen / in to lean
• Sometimes goes backwards adjusting air
• Do not bottom
• Spring to help maintain adjustment

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Idle problems

• Plugged passages
• One side of two barrel on two plane manifold
• Failing solenoids
• Plugged air bleeds
• Vacuum leak diagnosis
• Pinch off vacuum hoses
• Propane method
• Carb spray method

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Idle adjustments

• Idle speed
• Position of throttle plates in bore
• Usually turning appropriate screw
• Curb idle
• Idle stop solenoids
• Electric or vacuum
• Idle mixture
• Usually turning screw
• Screw anti-tamper devises
• Plug removal
• Limiter caps
• Double d heads
• Hex heads
• Allen heads
• Preset by bottoming screw and turning out
• If two / four barrel keep both screws the same
• Adjust to factory recommendations
• Adjustments
• Explain stoichiometrics

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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
• Problems
• Plugging
• Lean hesitation
• More noticeable on smaller displacement engines
• Usually older people complaints

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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
• Vacuum
• Problems and adjustments
• Jets change / do not drill (causes swirling)
• Rod changing
• Float level increases
• Usually only circuit found in the secondary
  bore(s)

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Power circuit

• Purpose to enrichen mixture through main circuit
  for increased power under load
• Components and operation
• Some have jet
• Calibrated size with stamp number
• May incorporate spring and valve
• Manifold vacuum low at w.O.T.
• Diaphragm type (holleys)
• Power piston on jet
• Power piston with rods
• Power piston return springs
• Problems and adjustments
• Vacuum piston stuck on or off
• Rod type
• Valve type
• Diaphragm rupturing
• Possible backfiring
• Can be accessed externally
• High overlap cams causing lower intake vacuum

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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
• Components and operation
• Plunger in machined bore (hooked to linkage)
• Diaphragm (hooked to linkage by cam)
• Check balls, weights and springs
• Pump discharge jet
• Duration and return springs
• Dual well accel. Pumps
• Toyotas cold accel enrichment
• Inspection of system
• Look for solid stream
• No external leaks
• Problems and adjustments
• Plunger
• Cup material deteriorating
• Rubber and oxygenated fuels
• Viton
• Carb cleaners swell cups

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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 not running
  efficiently cold needing more idle speed to run
• Components and operation
• Plate or butterfly
• Off centered to help opening
• Draws fuel from all points when closed including
  air bleeds
• Increases venturi vacuum under plate when closed
• 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
• Delay valves
• Can control secondary air valve
• Coil
• Electric
• Charging system output controlled
• Oil pressure controlled

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Secondary(s)

• Purpose to increase volume of air fuel mixture
  under load
• Usually consists of a main type circuit
• May also have idle, and accel. Pump circuits
• Can be mechanical or vacuum
• Can have air valves controlled by pulloffs
• Usually have a lock out mechanism
• Progressive vs normal

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