Accelerator Pump

An accelerator pump is a small diaphragm or plunger pump built into some carburettors. When the throttle is snapped open, air speeds up faster than fuel can follow through the jets, so the mixture briefly turns lean. The pump squirts a short shot of fuel into the intake to cover that moment.

Accelerator pumps are common on four-stroke motocross and enduro carburettors, such as the Keihin FCR. Most two-stroke carburettors manage without one.

If the bike hesitates or bogs when you crack the throttle, check that the pump squirts a strong, immediate jet, that the nozzle is clean and that the diaphragm is not torn. The timing and length of the squirt can be adjusted on many carburettors.