Fuel tap

How it works

The fuel tap, fitted to most carburettor bikes, controls the flow of fuel from the tank to the carburettor. It often has three positions: ON (normal), RES (reserve, which draws from lower in the tank) and OFF or PRI (prime). On many bikes it is a vacuum tap: a diaphragm opens it only when the engine runs, using intake vacuum carried by a small hose. A filter screen inside the tank catches debris. Over time the screen clogs, the diaphragm splits or hardens and the seals leak: the engine runs out of breath or dies as if out of fuel with a full tank, or fuel drips from the tap. A split vacuum hose stops the tap from opening. A rebuild kit (seals, diaphragm) often brings the tap back to life; otherwise it is replaced.

Maintenance tips

• ✓If the engine dies as if out of fuel with a full tank, try PRI/RES and check the vacuum hose.
• ✓Clean the tap's filter screen whenever the tank is removed.
• ✓A drip of fuel at the tap is a fire risk: replace the seals without delay.
• ✓Vacuum tap: leave it on ON; PRI is only for priming.
• ✓Always work with the tank empty or shut off, in a ventilated area, away from any flame.