The engine starts well from cold but is hard to restart after a ride, for example after stopping for fuel. A hot engine needs only a little extra fuel. Too much fuel (choke left on, a float that leaks, a dripping injector) floods it. Heat also exposes weak electrical parts: a coil or pick-up with a cracked winding can stop working when hot and recover when cool. A third group is mechanical: valves set too tight can close less well once the engine expands.
Is it safe to ride?
Yes, with care. If the engine also cuts out while riding when hot, do not ride in traffic until it is fixed: see Ignition cuts out when hot and returns when cool.
Quick checks first
- Carburettor: is the choke fully OFF? Do not use the choke on a hot engine.
- Fuel-injected: do not open the throttle while starting unless your manual says so (K-Light 125 manual, PDF page 277).
- Is there a strong smell of fuel after a failed start?
- Does it start normally once the engine has cooled down?
- Has the engine been overheating?
Diagnose it step by step
1Is the choke fully OFF on a carburettor bike?
- No: push it off. If it will not go off, see Choke is stuck on or will not work.
- Yes, or fuel-injected: go to step 2.
2After a failed hot start, is the plug wet and smelling of fuel?
- Yes: it is flooding. Clear a flooded engine. Then find why: a leaking float needle (Check the carburettor float and needle valve) or a dripping injector (Fuel injector suspected blocked or leaking).
- No, the plug is dry: go to step 3.
3Straight after a failed hot start, is there a strong spark on a spark tester?
- No, but it returns when cold: the coil or pick-up fails when hot. Test the ignition coil and plug cap and Test the ignition pick-up. Test them while hot.
- Yes: go to step 4.
4Fuel-injected: is there a temperature sensor fault code, or does the engine start if you open the throttle a little?
- Yes: Check the engine sensors (TPS, MAP, temperature). A wrong temperature reading makes the ECU add cold-start fuel to a hot engine.
- No, or carburettor: go to step 5.
5Is the engine running very hot (fins blocked, low oil, lean mixture)?
- Yes: Air-cooled engine overheats.
- No: go to step 6.
6Are valve clearances and compression correct?
- Have them checked: Check and adjust valve clearance, Test engine compression. Do the compression test warm.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Choke left on or sticking | Choke lever or plunger not fully off | Check the choke or starter valve |
| Flooding from float or injector | Wet plug, fuel smell | Clear a flooded engine |
| Coil or cap failing when hot | No spark hot, spark cold | Test the ignition coil and plug cap |
| Pick-up failing when hot | Resistance changes a lot when hot | Test the ignition pick-up |
| Temperature sensor fault (EFI) | Fault code or wrong reading | Check the engine sensors (TPS, MAP, temperature) |
| Engine overheating | Hot smell, pinking, fins blocked | Check air cooling (fins, airflow, mixture) |
| Valve clearance too tight | Feeler gauge check | Check and adjust valve clearance |