The plug insulator is chalk white, glazed or blistered, and the electrodes may look burnt or have small metal beads. The plug and the combustion chamber are running too hot. MAVCC lists the causes as timing too far advanced, a blocked carburettor, intake air leaks, a plug of the wrong heat range and a lean mixture (too little fuel for the air). The K-Light 125 manual links a white plug to a lean mixture and the fuel pump (PDF pages 23 and 24). An engine that runs this hot can hole a piston.
Is it safe to ride?
With care and gently. Avoid full throttle, long climbs and heavy loads until fixed. Stop at once if you hear pinking (a rattle when accelerating) or the engine loses power when hot.
Quick checks first
- Is the plug the type your manual names? A plug that is too "hot" overheats. K-Light 125: D7RTC.
- Look at the intake rubber for cracks and loose clamps.
- Is the air filter fitted, and is the air box lid on? (A missing filter lets in too much air.)
- Has the exhaust or air filter been changed to a free-flow type?
- Is fuel flow strong, and is the fuel filter clean?
Diagnose it step by step
1Is the plug the correct type and heat range?
- No: fit the correct plug. Remove, check, gap and fit a spark plug.
- Yes: go to step 2.
2Spray a little water around the intake rubber with the engine idling. Does the idle change?
- Yes: air leak. Find an intake air leak.
- No: go to step 3.
3Has the exhaust or air filter been changed, or is the air box open?
- Yes: refit standard parts, or have the fuelling matched by a workshop. Do not guess jet sizes.
- No: go to step 4.
4Is fuel delivery good: strong flow from the tap and a clean filter (carburettor), or a running pump with no fault code (fuel injection)?
- No: Replace the fuel filter, Check the fuel pump (fuel injection), or see Engine stops and only runs on reserve or after a rest.
- Yes: go to step 5.
5Carburettor: are the main jet and passages clean?
- No: Clean the carburettor.
- Yes, or fuel-injected: go to step 6.
6Are the cooling fins clean and the oil level correct?
- No: Check air cooling (fins, airflow, mixture).
- Yes: go to step 7.
7Is the ignition timing correct?
- Have it checked: Check the ignition timing. A wrong aftermarket CDI can advance the timing too far.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Wrong plug heat range | Plug code differs from manual | Remove, check, gap and fit a spark plug |
| Intake air leak | Spray test | Find an intake air leak |
| Modified exhaust or air filter | Non-standard parts fitted | Refit standard parts or have fuelling set |
| Blocked fuel filter or weak pump | Low flow; fault code | Replace the fuel filter |
| Blocked main jet | Weak at full throttle | Clean the carburettor |
| Blocked cooling fins or low oil | Visual, dipstick | Check air cooling (fins, airflow, mixture) |
| Timing too far advanced | Timing light | Check the ignition timing |