HRR Web Exclusive: 14 Typical Carb Symptoms & Their Causes

You’ve just finished a carburetor install for your customer and now the engine won’t start. What’s the problem? In this Web Exclusive, Hotrod & Restoration Technical Editor Mike Mavrigian discusses this and 13 other common problems associated with carburetors, and offers possible causes to help you with diagnose and solve the issue.

Symptom: Engine won’t start
Causes: Insufficient fuel in tank, don’t rely on your fuel gauge alone, especially with old or reconditioned sender and/or fuel gauge
No or insufficient fuel delivery, check pump and filter
Squirters clogged
Incorrect ignition timing
Faulty coil or coil wire

Symptom: Fuel runs out of the vent tube
Causes: Float level too high
Fuel pressure too high
Dirty needle and seat
Stuck float

Symptom: Fuel leaks externally from throttle shaft
Causes: Float level too high
Fuel pressure too high
Misadjusted butterfly

Symptom: Engine runs rich at idle
Cause: Timing not advanced far enough
Mixture too rich, turn screws in
Float level too high
Fuel pressure too high
Butterfly misadjusted

Symptom: Engine stumbles under light acceleration
Causes: Mixture too lean
Float level too low
Butterfly misadjusted
Faulty/inadequate accelerator pump operation, the most-common cause if an inadequate accelerator pump shot: Look at the discharge nozzle and verify that you have a good stro9ng pump shot. Inspect the pump diaphragm for a hole or tear. Make sure that the pump passage is clean. Check pump adjustment: open and hold the throttle to WOT and push the pump arm down. Adjust the pump override spring to obtain 0.015” clearance between the pump arm and lever.
If you have a stumble with no black smoke from the exhaust, you need to increase the shooter size. If it stumbles and blows black smoke, you need to decrease shooter size.

Symptom: Engine stumbles under hard acceleration
Causes: Squirters too small
Butterfly misadjusted

Symptom: Engine tends to surge at partial throttle
Causes: Idle mixture too lean, open screws
Butterflies misadjusted
Float level too low
Jets too small

Symptom: Engine appears to be rich and produces black smoke at exhaust under hard acceleration
Causes: Float level too high
Fuel pressure too high
Jets too large

Symptom: Engine will not return to idle, engine speed too high
Causes: Timing not advanced enough
Butterfly misadjusted
Weak/missing return spring
Worn/sticking throttle shaft

Symptom: Engine backfires through the carb
Causes: Idle mixture too lean
Squirters too small
Jets too small
Float level too low

Symptom: Engine backfires through the exhaust
Causes: Float level too high
Fuel pressure too high
Jets too large
Butterfly misadjusted

Symptom: With transmission in park, the secondaries will not open as the engine is revved
Causes: Note: the secondaries will not open by free-revving the engine as a load is required for the secondaries to open. To determine if the secondaries do open, install a paper clip onto the secondary diaphragm rod and push it up against the bottom of the secondary diaphragm housing. Test drive the vehicle under load. When you return and park the vehicle, inspect the position of the paper clip on the rod. If the clip is lower on the rod, this tells you that the secondaries did open and how far they opened. This is useful in determining if you need a lighter or heavier secondary spring.

Symptom: Puddle of fuel is pooled on the intake manifold after the vehicle has been driven, parked and shut off for a while
Causes: The most-common cause, aside from an external fuel leak, is fuel percolation. This can occur when the engine is shut off and engine temperature rises, causing the fuel in the bowl to boil and leak out of the boosters. Make sure that fuel level is not too high. Installing a phenolic heat spacer between the carb and manifold may cure the problem.

Symptom: Vacuum secondary carb-equipped engine bogs when the secondaries open
Causes: Bogging and hesitation can be caused by the secondaries opening too quickly. Installing a heavier secondary spring may cure this. If the engine is sluggish in response at WOT, this may indicate that the secondaries aren’t opening soon enough, which may require installing a lighter spring.

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