Carburetor Troubleshooting

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  • 8/4/2019 Carburetor Troubleshooting


    Cold StartingProblem


    Engine cranks but willnot start. 1) Choke not closing. 1) Inspect choke adjustment andfor something binding. Adjust ifnecessary.

    2) Choke linkagebinding.

    2) Lube with WD-40 and checkfor something bent. Adjust ifnecessary.

    3) No gas in carb. 3) Check fuel delivery. Look forplugged filter or clogged lines,bad pump, stuck needle & seat,and fuel pressure.

    4) Accelerator pumpdefective or blown out.

    4) Replace the pump. Problem isusually caused by bad gas, dirtin gas, or vacuum leak or ignitionproblems causing enginespitback.

    5) No spark or engineproblems such as badcompression.

    5) Diagnose & correct theproblem.

    Engine starts, then dies

    within a few seconds.

    1) Choke not closing


    1) See notes above. Adjust

    choke if necessary.

    2) Big vacuum leak onengine somewhere.

    2) Use vacuum gauge to check.Fix the leak. You may have putthe base gasket on wrong or it isthe wrong one for this carb &engine combination.

    3) Choke pull off settingincorrect.

    3) Adjust to factory specs. Carbcan get bumped in shipping ortransportation and canaccidentally change the setting.

    4) Fast idle RPM settoo slow.

    4) Adjust to recommended RPM.

    5)Low fuel delivery. 5) Correct delivery to carb.Usually it is a plugged up filter.

    6) Electrical or 6) Do complete tune up &

  • 8/4/2019 Carburetor Troubleshooting


    compression problemson the engine.

    diagnosis. Fix the problemsfound.

    7) Float level set verylow.

    7) Check & adjust the float levelto factory specs.

    8) No electricity up toidle solenoid, or noground.

    8) Check for power & ground.Correct the problem.

    9) Defective idlesolenoid.

    9) Replace it.

    Engine normally startsOK but then dies backingout the driveway or atthe first stop sign. After

    that it runs OK.

    1) Choke not set tightenough.

    1) Set choke a little tighter & tryit.

    2) Choke pull-off

    opening too much.

    2) The setting varies depending

    upon the exact application. Onecarb may fit several applications.Set the pull-off so the choke is alittle tighter.

    Engine starts OK,increases RPM then getstoo slow with lots ofblack smoke.

    1) Choke set too tight. 1) Adjust a little (1/8") looser.

    2) Pull-off set too tight. 2) Adjust so it is open a littlemore.

    3) Slow flooding. 3) Fix cause of flooding. (see"flooding" section coming soon)

    4) Float level very high. 4) (Rare) Set to factory specs.

    5) Power valve blown.(only happens on Ford& Holley)

    5) Replace the power valve.

    6) Pull-off diaphramblown.

    6) Caused by installer allowingengine to spit-back up throughcarb. Replace the pull off.

    7) (on Carter) Pumpblown out

    7) Same cause as #6 above.Replace the pump.

    8) Sunk float. 8) Same cause as #6 above.Replace the float.

    9) Choke spring maybe backwards and isgetting tight when

    9) Remove choke cover, cool offchoke, reverse spring, reinstallcover and set tension to factory

  • 8/4/2019 Carburetor Troubleshooting


    heating up instead ofloose.


    Engine starts, then racesfor a few seconds and

    then dies every time.

    1) Big vacuum leaksomewhere.

    1) Correct the vacuum leak.Make sure you haven't forgotten

    to hook up a hose somewhere.Base gasket may be wrong oneor on wrong.

    Warm StartingProblem

    . . . . . . .

    CONDITION POSSIBLE CAUSE CORRECTIONEngine cranks butwill not start.

    1) Carburetorflooding.

    1) See section on "flooding" for details onhow to correct.

    2) Choke is closedwhen engine is hot.

    2) Find & fix cause for choke stayingclosed. Look for no heat source, spring inbackwards, or something jammed or bent.

    3) No spark. 3) Do complete tune up.

    4) No compression. 4) Diagnose cause of no compression &fix.

    5) No fuel 5) Check fuel delivery volume andpressure. Look for clogged lines, filter, orpump. Check for kinked or swollen fuellines.

    6) No air. 6) Check for clogged air filter, especially

    after driving through muddy or dusty area.

    7) Too much air 7) Look for big vacuum leak, such asbroken hose, blown gasket, bad powerbrake diaphram, bad PCV valve.

    Engine starts, thendies within a few

    1) Choke is stayingclosed

    1) Diagnose and fix choke problem.

  • 8/4/2019 Carburetor Troubleshooting


    seconds. 2) Flooding 2) See section on flooding.

    3) Power valveblown.

    3) Replace power valve.

    4) Venting systemfailure. 4) Check out entire fuel system ventingsystem, inluding the vent valve on thecarb, the charcoal canister, all hoses &check valves in the system, and anysolenoids that trigger the vent system tooperate.

    5) Idle jet pluggedup with dirt.

    5) Clean out the idle jet and any other dirtin the carb.

    6) Idle air bleedplugged up or


    6) Check idle air bleed. Clean or replaceas necessary.

    7) Idle cut-offsolenoid notworking.

    7) Check idle solenoid, especially forpower to it and ground to it, replacesolenoid if necessary.

    Engine starts, thenraces for a fewseconds and thendies.

    1) Big vacuum leaksomewhere.

    1) Find the vacuum leak & fix it. Also lookfor wrong base gasket or one that isinstalled wrong. If engine has been spittingback, it may have blown out the basegasket or a gasket in the carb.

    Engine starts OK,but then get realrough. Lots ofblack smoke.

    1) Power valveblown out byspitback up throughcarb.

    1) Replace the power valve.

    2) Slow flooding. 2) See flooding section.

    3) Float sunk.(usually caused byspitback up throughcarb.)

    3) Replace float.

    4) Venting systemproblem.

    4) Check entire venting system & fix.

  • 8/4/2019 Carburetor Troubleshooting


    Cold EngineDrivabilityProblem

    . . . . . . .

    CONDITION POSSIBLE CAUSE CORRECTIONEngine stalls whentransmission is put intogear.

    1) Incorrect choke pull-offadjustment.

    1) Readjust the pull-off orreplace if necessary.

    2) Fast idle RPMincorrect (too slow)

    2) Speed up the fast idle tofactory specs.

    3) Engine running toolean because of vacuum

    leak or dirty jet.

    3) Check for vacuum leak.Flow test carb to check jetting.

    Hesitation, stalling,stumbling, flatspot, ordeadspot duringacceleration: Backfiringor spitback up throughcarb.

    1) Vacuum leak. 1) Check for vacuum leak &fix it.

    2) Ignition timing retardedtoo far.

    2) Reset timing.

    3) Accelerator pumpnozzle has dirt in it.

    3) Clean out the nozzle tip.

    4) Accelerator pump cup

    swollen up from contactwith bad gas orchemicals.

    4) Replace the pump cup.

    5) Economizer jet toosmall or partly blocked.

    5) Clean out economizer jet &check the size.

    6) Choke pull-off opentoo far.

    6) Adjust the pull-off tighter.

    7) Secondary throttleplates not closing all the


    7) Fix it.

    8) Vacuum hoses hookedup wrong.

    8) Connect up right. Beespecially careful of the EGR& Dist. connections:sometimes the pipe locationsare reversed on Rochesters.

  • 8/4/2019 Carburetor Troubleshooting


    9) Idle jet partly blockedwith dirt.

    9) Clean out the jet & anyother dirt that is in there.

    10) Distributor timing notadvancing properly.

    (worn breaker plate, wornshaft, pin hole indiaphram, crack in hose,etc.)

    10) Check distributor and allrelated systems carefully.

    Replace defective parts.

    Hesitation, deadspot orstalling after first mile ofwarmup.

    1) Defective electricassist on choke

    1) Replace it.

    2) Defective acceleratorpump (low output).

    2) Replace it.

    3) Float level setting very


    3) Adjust to factory specs.

    4) Bad ignitioncondenser.

    4) Replace it.

    Periodic backfiring withblack exhaust smoke:Deadspot, flatspot,hesitation, stumbling,backfiring.

    1) Plugged heatcrossover system inmanifold.

    1) Inspect and cleanpassages in intake manifoldand heads. Test heat riservalve, replace if defective.

    2) Defective source of hotair up to the carburetor.

    2) Check and replace asnecessary: heat shroud duct,

    temperature sensor, vacuumdoor motor, manifold vacuumsupply.

    Warm EngineDrivability


    . . . . . . .

    CONDITION POSSIBLE CAUSE CORRECTIONHesitation under 1) Vacuum leak 1) Inspect hoses. Route and lead the

  • 8/4/2019 Carburetor Troubleshooting


    light throttle:Deadspot &stumble.

    somewhere, orhose off or hookedto wrong vacuumfitting.

    hoses correctly. Look for leak because ofwrong base gasket or it was installedupside down.

    2) Acceleratorpump problems.

    2) Inspect and adjust pump stroke, pumpplunger, discharge nozzles and checkvalves. Inspect the accelerator pump, lookfor swollen pump cup.

    3) Float level setvery low.

    3) Set float to factory specs.

    4) Ignition timingretarded.

    4) Set to factory specs. Make sureadvancing correctly.

    5) Dirty idle jet or

    economizer jet.

    5) Inspect idle jets. Clean as necessary.

    6) Idle speed settoo fast & mixture istoo lean(common!).

    6) Richen up the idle mixture, reset idlespeed to factory specs, then lastly resetthe mixture using the lean drop method.

    7) Idle cut-offsolenoid notworking, or nopower to it or noground to it.

    7) Inspect & fix as necessary.

    8) Frozen orbinding heated airinlet (stuck in fullhot or full coldposition).

    8) Inspect & fix as necessary.

    9) E