Ethanol is a type of alcohol that is blended into petrol in many places. A small share is common, and many bikes are made for it. Problems appear on older bikes, on bikes not made for it, and after long standing. Alcohol holds water, so the fuel can take up moisture. It can also soften or harden old rubber parts, and it gives less energy per litre, so fuel use goes up a little.
Is it safe to ride?
Ride with care. If the engine stalls, runs lean or leaks fuel, stop. A lean engine runs hot and can be damaged.
Quick checks first
- Check your bike's manual for the highest alcohol share it allows. Some makers allow a small share only, and some warn against methanol.
- Look at the fuel in a clear jar. A cloudy layer or water at the bottom is a warning.
- Smell and look at fuel hoses and the tap for swelling, softness or cracks.
- Note whether the trouble began after a change of fuel station.
Diagnose it step by step
1Is the engine hard to start when the weather is cold?
- Yes: fuels with a high alcohol share vaporise poorly in the cold. Try a fuel with a lower share. See Hard to start when the engine is cold.
- No: go to step 2.
2Does a shake test show alcohol above the share your manual allows?
- Yes: drain and refill. See Test fuel for alcohol with a shake test and Drain old or contaminated fuel.
- No: go to step 3.
3Is there water at the bottom of the tank or in the bowl?
- Yes: drain the tank, bowl and fuel line. See Water or dirt in the fuel.
- No: go to step 4.
4Are rubber parts hard, swollen or leaking, or is the engine running lean?
- Yes: replace damaged hoses, seals and the needle and seat. See Fuel is leaking or dripping and Engine runs hot and lean.
- No: see Old or stale fuel in the tank.
Two-stroke bikes with pre-mix oil. Water settles to the bottom of the tank and alcohol joins it. That layer has no oil in it. If it reaches the engine, there is no lubrication for the crank and piston, and the engine can seize. Drain any water layer and use fuel that is sound.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Water drawn into the fuel | Cloudy fuel, water layer in a jar | Drain old or contaminated fuel |
| Alcohol share too high for the bike | Shake test above the manual limit | Use fuel with less alcohol |
| Old rubber hoses and seals affected | Hard, swollen or soft parts | Replace with parts rated for alcohol blend |
| Lean running from a weaker fuel | Pinking, hot engine, white plug | Clear carburettor jets and passages |
| Rust in steel tank | Rust flakes in the filter | Remove rust from a fuel tank |