Backfire (Afterfire)

A backfire is a loud bang or popping caused by fuel igniting outside the combustion chamber. Strictly, a bang in the exhaust is an afterfire, while a backfire goes back through the intake, but riders use the word for both.

Popping in the exhaust when closing the throttle is usually caused by a lean mixture at small throttle openings, an air leak at the exhaust joints, or fresh air let in by the emissions system (PAIR valve). Unburnt fuel then meets oxygen and hot gas in the pipe. Loud bangs can also come from misfires or a very rich mixture.

Backfiring through the intake points to a lean mixture, wrong ignition timing or a valve that is not sealing. Check exhaust gaskets, intake rubbers, jetting or fuel mapping and valve clearances.