A bad accelerator pump in a carburetor can cause several driveability issues — especially during throttle transitions. This pump is responsible for delivering an extra shot of fuel when you press the gas pedal. If it’s not working correctly, the air/fuel ratio leans out temporarily, causing poor performance.
Symptoms of a Bad Accelerator Pump:
1. Hesitation or Stumble on Acceleration
When you press the throttle, the engine hesitates, stumbles, or even bogs down before recovering.
Most noticeable when accelerating from a stop or when quickly increasing throttle.
- This can also be caused by the distributor advance not working correctly.
2. Flat Spot in Throttle Response
A noticeable "dead zone" or delay in power when you start to accelerate.
The engine feels like it's lagging or not responding right away.
3. Backfiring Through the Carburetor
A lean condition from a failed pump can cause a pop or backfire through the carb when the throttle is opened suddenly.
4. Poor Cold-Start Driveability
With no extra fuel delivered on cold starts, the engine may stall or stumble when driving cold.
5. Lack of Pump Shot (Visual Test)
With the engine off, remove the air cleaner and look into the carb.
Manually open the throttle — you should see a strong squirt of fuel into the venturi.
No squirt = bad pump (or blocked passage).
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Causes of Accelerator Pump Failure:
Dried or cracked pump diaphragm
Worn-out pump cup or plunger
Clogged pump passage or check valves
Improper linkage adjustment
Fix:
Clean or rebuild the accelerator pump circuit.
Replace the pump diaphragm or cup with a rebuild kit.
Adjust the linkage to ensure full travel.
