After riding for a while the engine stops, or misfires badly, then starts again after it has cooled for a time. Heat makes metal and plastic expand. A coil, pick-up or CDI with a hairline crack in its winding or circuit board can open up when hot and close again when cold. A coil can pass a cold resistance test and still fail when hot or under load. But fuel faults behave the same way: a blocked tank vent or a tired fuel pump also lets the engine run, stop, and recover after a rest. So test at the moment it fails.
Is it safe to ride?
No, not in traffic. A sudden cut-out can leave you stopped in the road. Ride only to test it, in a safe place.
Quick checks first
- When it stops, open the fuel cap: does air hiss in? If so, see Engine stops after a few kilometres and the cap hisses when opened.
- Fuel-injected: when it stops, turn the key OFF and ON. Does the pump run for its usual few seconds?
- Is the engine overheating (very hot smell, pinking, oil low)?
- Does the dashboard stay lit when it cuts? If everything goes dark, the fault is power supply, not ignition.
Diagnose it step by step
1When it cuts out, does the dashboard and every light go dark too?
- Yes: the main power supply is breaking: key switch, fuse, battery connection or main wire. See Electrical faults come and go.
- No: go to step 2.
2Does it restart at once if you open the fuel cap?
- Yes: blocked tank vent. Check the fuel tank vent.
- No: go to step 3.
3Straight after it cuts out, is there a spark on a spark tester?
- No spark when hot, spark when cold: an ignition part fails with heat. Go to step 4. Use Check for a spark safely.
- Good spark: the fault is fuel or overheating. Fuel-injected: Check the fuel pump (fuel injection). Overheating: Air-cooled engine overheats.
4While hot and failing, measure the pick-up resistance. Is it open or far from its cold value? (K-Light 125 cold value: 150 ± 10 Ω at 20 °C)
- Yes: replace the pick-up. Test the ignition pick-up.
- No: go to step 5.
5While hot and failing, does the coil primary or secondary read open, or does a known good coil keep it running?
- Yes: replace the coil. Test the ignition coil and plug cap.
- No: go to step 6.
6Are the connectors near the engine and under the seat tight and clean, or does wiggling them stop the engine?
- Faulty: Clean and seal electrical connectors.
- All good: the CDI or ECU is the suspect. Keep it shaded and check its mounting; then Diagnose a suspected CDI or ECU fault.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Blocked fuel tank vent | Hiss on opening cap; restarts at once | Check the fuel tank vent |
| Ignition coil failing hot | No spark hot; swap test | Test the ignition coil and plug cap |
| Pick-up failing hot | Resistance open or far off when hot | Test the ignition pick-up |
| Loose connector expanding with heat | Cut-out when wiggled | Clean and seal electrical connectors |
| Fuel pump weak when hot (EFI) | Good spark; no pump run | Check the fuel pump (fuel injection) |
| CDI or ECU failing hot | All else good; swap test | Diagnose a suspected CDI or ECU fault |
Reference values (K-Light 125)
| Item | Value | PDF page |
|---|---|---|
| Pick-up resistance | 150 ± 10 Ω at 20 °C | 87 |