|Cold Starting Problem
cranks but will not start.
||1) Choke not closing.
||1) Inspect choke adjustment for
something binding. Adjust if necessary.
|2) Choke linkage binding.
||2) Lube with penetrating oil
and check for something bent. Adjust if necessary.
|3) No gas in carb.
||3) Check fuel delivery. Look
for plugged filter or clogged lines, bad pump, stuck needle and seat, and fuel pressure.
|4) Accelerator pump defective
or blown out.
||4) Replace the pump. Problem is
usually caused by bad gas, dirt in gas or vacuum leak or ignition problems causing engine spitback.
|Engine starts, then dies
within a few seconds.
||1) Choke not closing properly.
||1) See notes above. Adjust choke if necessary.
|2) Choke setting incorrect.
||2) Adjust to factory specs. Carb can get bumped
in shipping or transportation and can accidentally change the setting.
|3) Fast idle RPM set too slow.
||3) Adjust to recommended RPM.
|4) Low fuel delivery.
||4) Correct delivery to carb. Usually it is a
plugged up filter.
|5) Electrical or compression problems on the
||5) Do complete tune up and diagnosis. Fix the
|6) Float level set very low.
||6) Check and adjust the float level to factory
|7) No electricity up to idle solenoid, or no
||7) Check for power and ground. Correct the
|8) Defective idle solenoid.
||8) Replace it.
normally starts OK but then dies backing out the driveway or at the first stop sign. After that it runs
||1) Choke not set tight enough.
||1) Set choke a little tighter
and try it.
|2) Choke pull-off opening too
||2) The setting varies depending
upon the exact application. One carb may fit several applications. Set the pull-off so the choke is a
|Engine starts OK,
increases RPM then gets too slow with lots of black 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 little more.
|3) Slow flooding.
||3) Fix cause of flooding. (See “flooding” section
|4) Float level very high.
||4) (Rare) Set to factory specs.
|5) Pull-off diaphram blown.
||5) Caused by installer allowing engine to
spit-back up through carb. Replace the pull off.
|6) Sunk float.
||6) Same cause as #5 above. Replace the float.
|7) Choke spring may be backwards and is getting
tight when heating up instead of loose.
||7) Remove choke cover, cool off choke, reverse
spring, reinstall cover and set tension to factory specs.
cranks but will not start.
||1) Carburetor flooding.
||1) See section on “flooding”
for details on how to correct.
|2) Choke is closed when engine
||2) Find and fix cause for choke
staying closed. Look for no heat source, spring in backwards, or something jammed or bent.
|3) No fuel.
||3) Check fuel delivery volume
and pressure. Look for clogged lines, filter or pump. Check for kinked or swollen fuel lines.
|4) No air.
||4) Check for clogged air
filter, especially after driving through muddy or dusty area.
|5) Too much air.
||5) Look for big vacuum leak,
such as broken hose, blown gasket, bad power brake diaphram, bad PCV valve.
|Engine starts, then dies
within a few seconds.
||1) Choke is staying closed.
||1) Diagnose and fix choke problem.
||2) See section on flooding.
|3) Venting system failure.
||3) Check out entire fuel system and venting
system; including the carburetor vent valve, the charcoal canister, all hoses and check valves in the
system and any solenoids that trigger the vent system to operate.
|4) Idle jet plugged up with dirt.
||4) Clean out the idle jet and any other dirt in
|5) Idle air bleed plugged up or missing.
||5) Check idle air bleed. Clean or replace as
|6) Idle cut-off solenoid not working.
||6) Check idle solenoid, especially for power to
it and ground to it, replace solenoid if necessary.
starts OK, but then get real rough. Lots of black smoke.
||1) Slow flooding.
||1) See flooding section.
|2) Float sunk. (usually caused
by spitback up through carb.)
||2) Replace float.
|3) Venting system problem.
||3) Check entire venting system
|Engine stalls when
transmission is put into gear.
||1) Incorrect choke pull-off adjustment.
||1) Readjust the pull-off or replace if necessary.
|2) Fast idle RPM incorrect (too slow).
||2) Speed up the fast idle to factory specs.
|3) Engine running too lean because of vacuum leak
or dirty jet.
||3) Check for vacuum leak. Flow test carb to check
Hesitation, stalling, stumbling, flatspot, or deadspot during acceleration: Backfiring or spitback up
||1) Vacuum leak.
||1) Check for vacuum leak and
|2) Accelerator pump nozzle has
dirt in it.
||2) Clean out the nozzle tip.
|3) Accelerator pump cup swollen
up from contact with bad gas or chemicals.
||3) Replace the pump cup.
|4) Economizer jet too small or
||4) Clean out economizer jet and
check the size.
|5) Choke pull-off open too far.
||5) Adjust the pull-off tighter.
|6) Secondary throttle plates
not closing all the way.
||6) Fix it.
|7) Idle jet partly blocked with
||7) Clean out the jet and any
other dirt that is in there.
|Hesitation, deadspot or
stalling after first mile of warmup.
||1) Defective electric assist on choke.
||1) Replace it.
|2) Defective accelerator pump (low output).
||2) Replace it.
|3) Float level setting very low.
||3) Adjust to factory specs.
Periodic backfiring with black exhaust smoke: Deadspot, flatspot, hesitation, stumbling, backfiring.
||1) Plugged heat crossover
system in manifold.
||1) Inspect and clean passages
in intake manifold and heads. Test heat riser valve, replace if defective.
|2) Defective source of hot air
up to the carburetor.
||2) Check and replace as
necessary: heat shroud duct, temperature sensor, vacuum door motor, manifold vacuum supply.
|Warm Engine Driveability Problem
|Hesitation under light
throttle: Deadspot & stumble.
||1) Accelerator pump problems.
||1) Inspect and adjust pump stroke, pump plunger,
discharge nozzles and check valves. Inspect the accelerator pump, look for swollen pump cup.
|2) Float level set very low.
||2) Set float to factory specs.
|3) Dirty idle jet or economizer jet.
||3) Inspect idle jets. Clean as necessary.
|4) Idle speed set too fast and mixture is too
||4) Richen up the idle mixture, reset idle speed
to factory specs, then lastly reset the mixture using the lean drop method.
|5) Idle cut-off solenoid not working, or no power
to it or no ground to it.
||5) Inspect and fix as necessary.
|6) Frozen or binding heated air inlet (stuck in
full hot or full cold position).
||6) Inspect and fix as necessary.
runs rough, lots of black smoke at idle.
||1) Choke staying closed or
||1) Fix choke or heat source.
|2) No electricity or heat
source to choke.
||2) Fix cause of no heat source
or electricity to choke.
|Hesitation under heavy
throttle: Deadspot & stumbles. May backfire or spitback.
||1) Defective accelerator pump.
||1) Look for dirt in pump nozzles, swollen cup
from bad gas, or check ball missing or stuck.
|2) Float level setting very low.
||2) Reset to factory specs.
|3) Secondary air valve set wrong.
||3) Check and adjust the secondary air valve
coming up to a stop sign, but idles ok.
||1) Bad throttle positioner or
bad vacuum source to it.
||1) Check throttle positioner
with a vacuum pump. Replace if defective. Replace any cracked hoses. Make certain that the vacuum hose
is connected to the correct pipe on carb or on the thermal switch. Make sure all related pipes have
|2) Idle speed and mixture
||2) Reset to factory specs. See
the adjustment and
installation instructions in owners manual.
|3) Loose or defective float
||3) Fix or replace the pin.
|Poor Performance or Gas Mileage
|No power or bad gas
||1) Clogged gas tank vent or fuel venting system.
||1) Remove gas cap and see if performance
improves. If so clean or replace the gas cap. Check the charcoal cannister, hoses to it and any check
valves. Check the electric vent valve on the carb if there is one, and make sure it is getting power at
the right time.
|2) Choke not opening.
||2) Fix choke or heat source problem.
|3) Secondary not opening.
||3) Check the lockout: secondaries won’t open
unless choke is coming off all the way. Check for sticky or bent shaft or linkage. If air valve type
(Rochester), check the spring tension, the plastic cam, and metering rods for dragging or sticking.
|4) Wrong main jets or rods.
||4) Check them. Replace if necessary.
|5) Dragging brakes.
||5) Fix brake problem.
|6) Float level much too high, or float partly
||6) Set to factory recommendations, and check
float weight, replace if necessary.