A vacuum fuel tap has a diaphragm that opens the fuel valve while the engine turns, and shuts it when the engine stops. The lever has ON (or MAIN), RES (reserve) and PRI (prime). PRI by-passes the diaphragm. If the engine runs only on PRI, or the tap lets fuel through when it should be shut, the diaphragm, the vacuum hose or the vacuum source is at fault.
Is it safe to ride?
Ride with care. Do not leave the lever on PRI while riding or parked, because fuel then flows with the engine stopped and can flood the carburettor. A leaking tap is a fire risk. Do not ride if fuel is dripping.
Quick checks first
- Set the lever to ON and check that the tank has fuel above the reserve.
- Look at the thin vacuum hose from the tap to the intake. Check it for splits, a loose end or a kink.
- Open the fuel cap. A blocked vent can also stop flow. See Check the fuel tank vent.
- Check whether fuel drips from the carburettor overflow after the bike is switched off.
Diagnose it step by step
1With the lever on ON, does fuel flow while the engine cranks?
- Yes, but not enough: clean the screens in the tank and tap. See Check the fuel tap, lines and flow.
- No: go to step 2.
2Does fuel flow on PRI?
- Yes: the diaphragm, the hose or the vacuum source is faulty. Go to step 3.
- No: the screens or the fuel line are blocked, or the tank is empty. See No fuel reaching the carburettor.
3Is there fuel in the vacuum hose, or is the hose loose or split?
- Yes: the diaphragm is torn. See Check a vacuum fuel tap and use the prime position.
- No: check the hose and the intake fitting for blockage and leaks. See Find an intake air leak.
4Does the tap let fuel through with the engine stopped?
- Yes: the valve seat or diaphragm is worn. Replace the tap, or fit an inline valve.
- No: the tap is working. Look at the float and needle in the carburettor.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Vacuum hose split, loose or blocked | Visible damage, no suction | Replace the hose |
| Diaphragm torn or hard | Fuel in the vacuum hose, flow only on PRI | Diaphragm kit or new tap |
| Blocked tap or tank screens | Weak flow on PRI too | Clean the screens |
| Worn valve in the tap | Fuel passes with the engine off | New tap or inline valve |
| Blocked tank vent | Cap hisses, flow stops | Check the fuel tank vent |
Many taps are fixed with small screws and can be opened. Riveted ones cannot. Parts for old taps are often not made, but aftermarket kits sometimes exist.