As fuel leaves the tank, air must come in to replace it. Air enters through a small vent: a hole in the fuel cap, or a vent hose from the tank or cap. Many newer bikes route the vent through a rollover valve and a charcoal canister (an emissions part that traps fuel vapour). If the vent blocks, a vacuum builds in the tank, fuel stops flowing and the engine dies after a few kilometres. When you open the cap you hear air rush in.
Safety
- Engine off and cool. No flames or sparks near the open filler.
- Never block or seal a vent to stop a smell or leak. A tank with no vent can starve the engine, and some tanks can deform.
You need
- A torch, a thin soft wire or nylon fishing line, and a hand pump or low-pressure air. Do not use high-pressure shop air into a tank.
Steps
- Listen test: after the engine has stopped from starvation, open the fuel cap slowly. A clear hiss of air going in means the vent is blocked.
- Ride test (rider): if the bike starts again after you open the cap and runs normally while the cap is loose, the vent is the cause. Only ride a short distance this way, at low speed, with the cap closed again before you move off on public roads.
- Find the vent. Look under the cap for a small hole or a one-way valve. Look under the tank for a thin hose that ends at the frame or goes to a canister.
- Clean the cap vent. Remove dirt and dried wax with a soft wire or fine line. Do not drill or enlarge the hole. If the cap has a valve inside, see Clean and lubricate the fuel cap.
- Check the vent hose. It must not be kinked, crushed under the tank, blocked by mud or pinched by a clip. Blow gently through it from the free end; air should pass.
- Bikes with a rollover valve or canister: the K-Light 125 has a rollover ("dump") valve and an evaporative canister. A blocked canister or valve gives the same symptoms. Check the hoses for kinks. A blocked valve or canister is replaced, not cleaned; ask a workshop if you are not sure which hose is which.
- Refit everything and close the cap.
Check your work
- Open the cap after a 10 to 20 km ride: little or no hiss.
- The engine no longer dies after a few kilometres.
If it did not fix the problem
- Engine still starves with a clear vent: see Engine stops and only runs on reserve or after a rest.
- Strong fuel smell around the cap or vent hose: see Strong smell of fuel.