Fuel pump

The fuel pump moves fuel from the tank to the engine. Fuel-injected motorcycles mostly use an electric pump inside the tank, which holds the fuel at the injectors at a set pressure – typically around 3 bar. When you switch the ignition on you hear the pump whirr for a couple of seconds as it pressurises the system.

On carburettor bikes fuel often simply flows by gravity, but where the tank sits lower than the carb (on many scooters and ATVs, for example) a diaphragm vacuum pump is used, driven by crankcase pulses or intake vacuum.

Signs of a weak pump are hard starting, loss of power under load and stalling. Check the fuel filter, fuses and relay too. Don't run the tank completely dry – the fuel cools the electric pump.