Rochester 4G Dead Spot on Acceleration – Troubleshooting Guide
Customer Issue:
After installing a new float in a Rochester 4G carburetor and completing a rebuild, the engine starts easily and runs well under normal conditions. However, a "dead spot" or hesitation occurs during quick acceleration—especially from a complete stop. Slow acceleration is smooth, and once the vehicle is moving, flooring the throttle causes no issues. The timing appears to be correct, and the carburetor is clean and properly adjusted.
Common Cause: Faulty Accelerator Pump Circuit
Your symptoms strongly point to a problem in the accelerator pump system. The accelerator pump provides a quick squirt of fuel when the throttle is suddenly opened. If this circuit isn’t functioning properly, the engine can hesitate, stumble, or even stall during quick acceleration.
What to Check First
1. Is the Check Ball Installed in the Pump Well?
Some Rochester 4G carburetors require a small check ball (usually aluminum) in the bottom of the accelerator pump well. Without this check ball, the pump will force gas back into the float bowl instead of out the main discharge.
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Look inside the pump well for two holes:
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One on the bottom (fuel intake)
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One slightly up the side (discharge)
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If there's a hole on the bottom, your carb needs a check ball in that location.
Test it:
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Remove the carb top and fill the float bowl with fuel. We use mineral spirits so we don't burn down our shop.
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From the bottom raise the pump. The suction from the pump and the weight of fuel will lift the check ball and fill the well. If fuel rises into the pump well, the check ball is working properly by allowing fuel in.
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If no fuel enters, the passage is likely clogged—common with ethanol residue. Try spraying carb cleaner into the passage to see if it goes through.
How to Clear Blockages
If the pump well doesn't fill, clean the passage:
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Use spray carb cleaner to clear any blockages.
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Ethanol fuel can leave behind residue that carburetor cleaner can't dissolve. A thin wire might work.
Testing the Accelerator Pump Discharge Circuit
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Remove the venturi cluster.
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Remove the main discharge T, spring, and check ball (this is the larger check ball).
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Fill the pump well with fuel, or mineral spirits.
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Press the accelerator pump down.
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Fuel should not return to the well.
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It should be forced out through the main discharge port.
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If fuel discharges properly, the passage is clear, and the intake check ball is sealing.
Reseating the Main Discharge Check Ball
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Place the check ball back into the main discharge hole.
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Use a drift punch to gently tap the ball to reform the seat.
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This helps the ball seal the hole properly.
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Do not strike hard—you could jam the ball.
Reassembly Tips
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Reinstall the check ball, spring, and T fitting. Stake the T with 1 stake. For the most part the gasket will keep it in place anyhow.
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Force air or spray carb cleaner through all venturi cluster passages.
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Don’t forget to replace the venturi gasket.
Final Test
After reassembly:
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Press the accelerator pump down fully.
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A strong fuel squirt should come out of the venturi discharge ports. In the case of a 4 jet there should be 2 strong streams.
If that’s working, your accelerator pump circuit is functioning, and the hesitation during acceleration should be resolved.
