When you stomp on the gas pedal of a carbureted engine, a rush of air enters the intake manifold. Because fuel is heavier than air, it lags behind, creating a sudden lean spot that makes the engine hesitate or stumble. The accelerator pump diaphragm prevents this. Connected to the throttle linkage, this rubber diaphragm compresses inside a small fuel chamber when you accelerate. This compression forces a quick squirt of raw fuel directly into the carburetor throat, bridging the fuel lag.
Rubber diaphragms degrade over time from exposure to modern ethanol gasoline. They dry out, crack, and leak fuel out of the side of the carburetor. This creates a major engine fire hazard and causes severe engine stumbling or backfiring during takeoffs.