Petrol does not keep for ever. Over weeks and months it loses the light parts that make cold starting easy, and it slowly forms gum and varnish. Fuel with ethanol in it also takes up water from the air. A bike that has stood for a long time is hard to start, idles badly and hesitates, and the gum can block carburettor jets, the float needle and injectors. The longer it stood, and the warmer the place, the worse it is.
Is it safe to ride?
With care, once it starts and runs smoothly. If it hesitates or stalls, fix it before riding in traffic.
Quick checks first
- How long has the bike stood with fuel in it?
- Does the fuel smell sour or like varnish rather than sharp petrol?
- Is it darker than fresh fuel in a clear jar?
- Was the tank half empty (more air, more condensation)?
- Carburettor: was the float bowl left full?
Diagnose it step by step
1Draw a sample into a clear jar. Is it darker than fresh fuel, cloudy, or does it smell sour or of varnish?
- Yes: the fuel is stale. Go to step 2.
- Water layer or dirt: see Water or dirt in the fuel.
- Looks and smells fresh: the fuel is not the problem. After storage, also see Will not start after standing.
2Drain the tank and float bowl and refill with fresh fuel of the correct grade.
- Drain old or contaminated fuel. Fit a new filter if it is dark: Replace the fuel filter.
3Does it now start and idle well?
- Yes: ride it for a full tank so fresh fuel flushes the system.
- No, carburettor: gum is in the jets and passages. Clean the carburettor. Pay attention to the pilot jet, which is the smallest and blocks first.
- No, fuel-injected: go to step 4.
4Fuel-injected: does the pump prime normally, and does it still run badly on fresh fuel?
- The injector may be gummed. The K-Light 125 manual says a blocked injector is cleaned with special equipment such as an ultrasonic cleaner. Workshop: Check the fuel injector. The pump may also have suffered if it ran dry or on gummed fuel.
5. Prevent it next time. Before storing a bike for more than a few weeks, follow Bring a bike back into use after storage in reverse: fill the tank to reduce air and condensation (a fuel stabiliser helps), and on carburettor bikes turn the tap OFF and drain the float bowl.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Fuel left for months | Sour smell, dark colour | Drain fuel |
| Gum in the pilot jet and passages | Poor idle after draining | Clean carburettor |
| Gummed injector | Poor running on fresh fuel | Injector check |
| Water from condensation | Water layer in jar | Water in fuel |