After trying to start, the bike smells strongly of petrol and the plug comes out wet. There is so much fuel in the cylinder that the spark cannot light it. Sometimes this is simply too many attempts with the choke on. If it keeps happening, fuel is getting in when it should not: a carburettor float or needle valve that does not shut, a leaking fuel tap, or a dripping injector. A missing spark also makes the engine flood, because fuel goes in on every turn but never burns.
Is it safe to ride?
Not until the fuel smell has gone and you know nothing is leaking. Petrol near a hot engine can catch fire. If fuel drips from the carburettor or engine, stop and fix it first.
Quick checks first
- Look for fuel dripping from the carburettor overflow pipe, the air box or the exhaust.
- Carburettor: is the choke still ON? Turn it OFF.
- Turn the fuel tap OFF while you work.
- Check the engine oil: does the level look too high or smell of petrol?
- No smoking or flames nearby.
Diagnose it step by step
1Is fuel dripping from the carburettor overflow pipe or the air box?
- Yes: the float or needle valve is not shutting. Go to Fuel pours out of the carburettor overflow pipe.
- No: go to step 2.
2Is the engine oil level higher than normal or does the oil smell of petrol?
- Yes: fuel has run into the crankcase. Do not start it. Change the oil before running the engine. See Oil smells of petrol or the level rises.
- No: go to step 3.
3Clear the flood with Clear a flooded engine. Does it start and run?
- Yes: go to step 4.
- No: go to step 5.
4Was the choke used on a warm engine, or did you crank many times?
- Yes: that was the cause. Use the choke only from cold and push it off as soon as the engine runs evenly.
- No: something is adding fuel. Check the fuel tap (Check the fuel tap, lines and flow), the float (Check the carburettor float and needle valve) and the choke (Check the choke or starter valve). On fuel injection: Check the fuel injector.
5With a dry plug, is there a strong spark?
- No: the missing spark caused the flooding. Go to No spark at the spark plug.
- Yes: fit a new plug (wet plugs often stay fouled): Remove, check, gap and fit a spark plug. Then try again.
6Is the air filter wet with fuel or badly blocked?
- Yes: dry or replace it. A blocked filter makes the mixture too rich. Clean or replace the air filter.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Choke on a warm engine or too many tries | Happens once, clears easily | Clear a flooded engine |
| Plug fouled by fuel | Plug stays wet or black | Remove, check, gap and fit a spark plug |
| No spark | No spark on tester | Check for a spark safely |
| Float or needle valve leaking | Fuel from overflow pipe | Check the carburettor float and needle valve |
| Fuel tap not shutting (gravity feed) | Floods while parked with tap ON | Check the fuel tap, lines and flow |
| Leaking injector (EFI) | Floods after parking; workshop test | Check the fuel injector |