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.