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Carburetion

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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 ... – PowerPoint PPT presentation

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Title: Carburetion


1
Carburetion
2
Purpose of carburetor
  • To provide proper ratio of air and fuel under
    every condition for good performance, economy and
    emissions

3
Fuel must be atomized to be vaporized
4
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

5
Multiple barrel carbs.
  • Primary vs. secondary barrels
  • Progressive vs. non-progressive
  • One - two - three - four

6
Carburetors have 2 or 3 main parts
  • Air horn, float bowl, throttle body

    or air horn, main body

7
Carb overhaul
  • Chemicals
  • Aerosol carb cleaner
  • Carburetor (cold parts) cleaner dip
  • Soak rinse
  • Blow out
  • Replace all gaskets
  • Perform all adjustments

8
Control of fuel to air ratio is primarily
controlled by seven different circuits
9
Circuits
  • Float circuit
  • Idle circuit
  • Transfer (off idle) circuit
  • Main circuit
  • Power circuit
  • Accelerator pump circuit
  • Choke circuit

10
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

11
Float level adjustment
  • Critical to all other circuits
  • Must be checked on overhaul

12
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

13
Problems with float circuit
  • Low float levels
  • Caused by improper adjustments
  • Will cause lean condition

14
Idle circuit / slow circuit
  • Purpose to supply engine with fuel at speeds too
    slow to operate main circuit

15
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

16
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

17
Idle problems
  • Plugged passages
  • Failing solenoids and/or circuits
  • Plugged air bleeds

18
Idle speed adjusment
  • Usually adjustments are accomplished by turning
    appropriate screw
  • Idle stop solenoids
  • Electric or vacuum
  • Curb idle means no load at stop

19
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

20
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

21
Transfer circuit problems
  • Plugging causing a lean hesitation
  • More noticeable on smaller displacement engines
  • Usually older people give complaints

22
Main circuit / main metering
  • Purpose to supply fuel to engine at cruise speeds

23
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

24
Problems and adjustments
  • Plugging jets and air bleeds
  • Jets change / do not drill (causes swirling)
  • Rod changing
  • Float level increases

25
Power circuit
  • Purpose to enrichen mixture through main circuit
    for increased power under load

26
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

27
Problems and adjustments
  • Vacuum piston stick on or off
  • Rod type
  • Valve type
  • Diaphragm rupturing
  • Possibly caused by backfiring
  • Often can be accessed externally

28
Accelerator pump circuit
  • Purpose to supply engine with shot of fuel on
    moderate - heavy acceleration
  • Necessary because fuel is heavier than air

29
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

30
Inspection of system
  • Look for solid stream
  • No external leaks

31
Problems and adjustments
  • Plunger
  • Cup material deteriorating
  • Diaphragm
  • Leaking externally due to oxygenated fuels

32
Problems and adjustments
  • Check balls leaking
  • Outlet ball sticking
  • Adjustments by hole in arms
  • Adjustments by cams

33
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

34
Components and operation
  • Plate or butterfly
  • Draws fuel from all points when closed including
    air bleeds
  • Increases venturi vacuum under plate when closed

35
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

36
Components and operation
  • Coil
  • Electric
  • Heated air
  • Divorced
  • Water
  • Fast idle cam

37
Adjustments
  • Choke coil
  • Alignment of index marks
  • Choke pulloff
  • Drill diameter
  • Angle gauge
  • Calibrated vacuum leaks
  • Fast idle speed

38
Problems
  • Choke coil not warming up
  • Plate sticking
  • Choke pulloff rupturing
  • Fast idle speed adj.

39
Secondary(s)
  • Purpose to increase volume of air fuel mixture
    under load

40
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
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