2. Carburetion

3. Purpose of carburetor sTsTo provide proper ratio of air and fuel under every condition for good performance, economy and emissions

4. Fuel must be atomized to be vaporized

5. Venturi principal sAsAir flowing through a tube will cause a pressure drop in that tube sAsAdding a restriction or two to a tube will cause a larger decrease in pressure (increase in vacuum) sTsThis vacuum is used to suck or draw fuel from the float bowl

6. Multiple barrel carbs. sPsPrimary vs. secondary barrels –P–Progressive vs. non-progressive sOsOne - two - three - four

7. Carburetors have 2 or 3 main parts sAsAir horn, float bowl, throttle body or air horn, main body

8. Carb overhaul sCsChemicals –A–Aerosol carb cleaner –C–Carburetor (cold parts) cleaner dip sSsSoak & rinse sBsBlow out sRsReplace all gaskets sPsPerform all adjustments

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

10. Circuits sFsFloat circuit sIsIdle circuit sTsTransfer (off idle) circuit sMsMain circuit sPsPower circuit sAsAccelerator pump circuit sCsChoke circuit

11. Float circuit : kind of fuel tank sPsPurpose: to store fuel for all other systems to work properly –I–If float system is malfunctioning, all other systems will be affected

12. Float level adjustment sCsCritical to all other circuits sMsMust be checked on overhaul

13. Problems with float circuit s High float levels –Can be caused by sunk float or float out of adjustment –Will cause rich condition s Needle and seat sticking / debris on seat –Usually causes flooding over –Sometimes can be flushed out

14. Problems with float circuit sLsLow float levels –C–Caused by improper adjustments –W–Will cause lean condition

15. Idle circuit / slow circuit sPsPurpose: to supply engine with fuel at speeds too slow to operate main circuit

16. Idle circuit components and operation sSsStarts through main jet sIsIdle restrictor / slow jet sAsAir bleeds sFsFuel cut / slow cut solenoid –S–Solenoid will have the ability to shut off all fuel in idle circuit

17. sIsIdle mixture screw –U–Usually out to richen / in to lean –D–Do not bottom –S–Spring on screw is to help maintain adjustment

18. Idle problems sPsPlugged passages sFsFailing solenoids and/or circuits sPsPlugged air bleeds

19. Idle speed adjusment sUsUsually adjustments are accomplished by turning appropriate screw sIsIdle stop solenoids –E–Electric or vacuum sCsCurb idle means no load at stop

20. Idle mixture sPsPerform only after checking for vacuum leaks sSsScrew anti-tamper devises sIsIf two / four barrel keep both screws the same sAsAdjust to factory recommendations

21. Transfer / off idle circuit sPsPurpose: supplies fuel at speeds too slow for main circuit sLsLimited by idle restrictor sEsEliminates one idle air bleed but adds volume to idle circuit sUsUsed on light acceleration

22. Transfer circuit problems sPsPlugging causing a lean hesitation sMsMore noticeable on smaller displacement engines sUsUsually older people give complaints

23. Main circuit / main metering sPsPurpose; to supply fuel to engine at cruise speeds

24. Components and operation sJsJet (stamped numbers on jet) sAsAtomizer / emulsion tube sAsAir bleed –A–Altitude compensators –A–At nozzle stops dripping, possibly due to high idle speed sDsDischarge nozzle sVsVenturi / boost venturis

25. Problems and adjustments sPsPlugging jets and air bleeds sJsJets change / do not drill (causes swirling) sRsRod changing sFsFloat level increases

26. Power circuit sPsPurpose: to enrichen mixture through main circuit for increased power under load

27. Components and operation s Most work on manifold vacuum –Manifold vacuum low at W.O.T. s Some have jet –Calibrated size with stamp number –May incorporate spring and valve s Diaphragm type (Holleys) s Power piston on jet s Power piston with rods s Power piston return springs

28. Problems and adjustments s Vacuum piston stick on or off –Rod type –Valve type s Diaphragm rupturing –Possibly caused by backfiring –Often can be accessed externally

29. Accelerator pump circuit s Purpose: to supply engine with shot of fuel on moderate - heavy acceleration s Necessary because fuel is heavier than air

30. Components and operation s Plunger in machined bore (hooked to linkage) s Diaphragm (hooked to linkage by cam) s Inlet and outlet check balls, weights and springs s Pump discharge jet s Duration and return springs

31. Inspection of system s Look for solid stream s No external leaks

32. Problems and adjustments s Plunger –Cup material deteriorating s Diaphragm –Leaking externally due to oxygenated fuels

33. Problems and adjustments s Check balls leaking s Outlet ball sticking s Adjustments by hole in arms s Adjustments by cams

34. Choke circuit s Purpose: to enrichen fuel mixture on cold conditions and to raise idle speeds cold s Fuel vaporizes slower cold causing much of the fuel to be wasted s Vehicle does not run as efficiently cold needing more idle speed

35. Components and operation s Plate or butterfly –Draws fuel from all points when closed including air bleeds –Increases venturi vacuum under plate when closed

36. Components and operation s Pulloff or brake –Purpose: to open choke plate a desired amount at first start to obtain a proper a/f ratio –Manifold vacuum controlled u Vacuum can be computer controlled u Vacuum can be tvs controlled –Can have a primary and secondary for two stages –May have a delay valve

37. Components and operation s Coil –Electric –Heated air –Divorced –Water s Fast idle cam

38. Adjustments s Choke coil –Alignment of index marks s Choke pulloff –Drill diameter –Angle gauge –Calibrated vacuum leaks s Fast idle speed

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

40. Secondary(s) s Purpose: to increase volume of air fuel mixture under load

41. Components and operation s Usually has only a main circuit s May also have idle, and accelerator pump circuits s Can be mechanical or vacuum controlled s Can have air valves controlled by choke pulloffs s Usually have a choke lock out