The bike starts and runs well, then after a few kilometres it slows and stops as if out of fuel. When you open the fuel cap you hear air rush in, and the engine then starts and runs again for a while. The tank vent is blocked. As fuel flows out, nothing replaces it, so a vacuum builds in the tank and holds the fuel back. The K-Light 125 manual's no-start chart lists a blocked tank cap air hole for this reason.
Is it safe to ride?
With care, for short distances, until the vent is cleared. The engine can stop suddenly in traffic. Never ride with the fuel cap left open.
Quick checks first
- Do you hear a clear hiss when you open the cap after the engine stops?
- Has the cap recently been changed, painted, or covered with a tank bag or cover?
- Is the vent hose under the tank (if fitted) kinked or blocked with mud?
- Has the bike been washed with a pressure washer near the cap?
Diagnose it step by step
1After the engine stops, open the cap slowly. Is there a hiss, and does the engine run again straight away?
- Yes: the vent is blocked. Go to step 2.
- No hiss: the vent is fine. Go to Engine stops and only runs on reserve or after a rest.
2Look under the cap. Does it have a vent hole or valve, and is it clean?
- Blocked with dirt, wax or paint: clean it gently: Clean and lubricate the fuel cap.
- No vent at all on a replacement cap: the wrong cap may be fitted, or your bike vents through a hose. Go to step 3.
3Does your bike have a vent hose from the tank or filler?
- Yes: follow it. It must not be kinked, crushed under the tank, pinched by a clip, or blocked with mud at its end. Check the fuel tank vent.
- No hose and a clean cap vent: go to step 4.
4Does your bike have an emissions vapour system (rollover valve and charcoal canister)?
- Yes: a blocked rollover valve or canister can cause the same vacuum. The K-Light 125 manual says a damaged rollover valve is replaced. Workshop, if you cannot identify the hoses.
- No: refit the cap and test for 10 to 20 km. If it still stops with a hiss, replace the cap.
5. After the fix, ride the same distance and open the cap. Little or no hiss means it is fixed. If the engine still stops without a hiss, check fuel flow: Check the fuel tap, lines and flow.