Petcock (fuel tap)

A petcock or fuel tap controls the flow of fuel from the tank to the carburettor. Common positions are OFF, ON, RES (reserve, which draws from a lower pick-up when the main supply runs dry) and on some bikes PRI (prime, which bypasses the vacuum valve to fill an empty carburettor).

Many bikes use a vacuum petcock that only opens when the engine is running. Fuel-injected bikes usually have no petcock; a pump in the tank and a low-fuel light do the job.

Typical faults are a leaking tap seal, a split vacuum diaphragm that lets fuel flood the engine, and a blocked filter screen. Turn the tap to OFF when parking a carburettor bike for long periods, and remember to switch back from RES after refuelling.