Thread: backfire at low rpm

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When you nail the throttle, the motor takes in a big gulp of air. You must introduce more fuel with this gulp or the motor goes lean, resulting in a backfire. Either the accelerator pump shooter is worn out or you have the shooter linkage in the wrong hole or the shooters are too small.

From the Edelbrock Owner's Manual:
When the throttle is opened rapidly, the air flow through the engine will increase immediately. The fuel, since it is much heavier than the air, will "lag" behind. This contributes to a temporary lean A/F condition. Regardless of cause, a solution is to temporarily enrichen the A/F Ratio by mechanically pumping a small quantity of fuel into the throat of the carburetor (Figure 7). The Edelbrock carburetor has a piston that draws fuel into the pump cavity past the plunger lip-seal when the throttle is closing (1). Upon opening, the lip-seal seats, allowing the plunger to force the fuel through another one-way valve, the Pump Discharge Ball and Weight (2), and the Pump Jets (3) into the primary throats.

The pump plunger is not driven directly by the throttle, but through an intermediate Pump Drive Spring (4) that extends the duration of the "pump shot" past the time the throttle stops moving. The Edelbrock Performer Series carburetor has an external pump lever with three hole locations for link attachment providing three distinct pump delivery curves. This is further explained in "Calibrating The Pump".

Go to www.edelbrock.com, on the left side of the page, click carburetors, then click Performer Series Owner's Manual.