The spark tester shows a spark, but it is thin, orange or red instead of bright blue-white, or it misses some turns. A weak spark may start the engine when conditions are easy but fail under load, at high revs, in rain or when cold. Colour is only a rough guide: a spark that looks fine in open air can still fail under compression. The K-Light 125 fault chart lists coil supply, ignition switch, CDI or ECU, trigger coil, battery charge and low-voltage wiring (PDF page 21).
Is it safe to ride?
With care. A weak spark can cause misfires and sudden loss of power. Avoid busy roads until it is fixed.
Quick checks first
- Is the battery fully charged? A weak battery gives a weak spark on battery-powered systems.
- Is the plug cap tight on the plug and lead, and dry inside?
- Is the coil firmly mounted to clean metal?
- How old is the plug?
Diagnose it step by step
1Is the battery charged, and does its voltage stay up while cranking?
- No: Test and charge a 12 V battery.
- Yes, or magneto bike: go to step 2.
2With a new, correctly gapped plug, is the spark now strong?
- Yes: the old plug was worn or fouled. Remove, check, gap and fit a spark plug. Find why it fouled: Read a spark plug.
- No: go to step 3.
3Are the plug cap and lead free of cracks, burn tracks and corrosion, and does the cap test good?
- No: replace the cap, or the coil if the lead is part of it. Test the ignition coil and plug cap.
- Yes: go to step 4.
4Are the coil earth, the main earth and the coil supply connectors clean and tight?
- No: Clean and tighten earth connections and Clean and seal electrical connectors.
- Yes: go to step 5.
5Does a known good coil give a strong spark?
- Yes: the old coil is weak. Replace it.
- No: go to step 6.
6Does the pick-up pass its resistance and peak voltage tests? (K-Light 125: 150 ± 10 Ω at 20 °C; peak above 1.7 V)
- No: Test the ignition pick-up.
- Yes: go to step 7.
7Magneto CDI: does the source coil in the stator test good?
- No: Test the stator (generator coils).
- Yes, or battery-powered: Diagnose a suspected CDI or ECU fault.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Worn or fouled plug | New plug gives a strong spark | Remove, check, gap and fit a spark plug |
| Low battery voltage | Voltage drops a lot when cranking | Test and charge a 12 V battery |
| Cracked or wet plug cap or lead | Tracking marks; cap test | Test the ignition coil and plug cap |
| Poor coil earth or connector | Corrosion; loose mount | Clean and tighten earth connections |
| Weak coil | Swap test | Test the ignition coil and plug cap |
| Weak pick-up or source coil | Low peak voltage | Test the ignition pick-up |
| CDI or ECU | All else good | Diagnose a suspected CDI or ECU fault |
| Worn points or a failed condenser (points ignition) | Pitted faces, wrong gap or heavy arcing | Inspect, clean and replace contact breaker points |
| Pick-up air gap too wide (electronic ignition) | Gap to the flywheel magnet checked with a non-magnetic gauge; use the value in your bike's manual | Test an ignition pick-up |
Reference values (K-Light 125)
| Item | Value | PDF page |
|---|---|---|
| Pick-up resistance | 150 ± 10 Ω at 20 °C | 87 |
| Pick-up peak voltage | above 1.7 V | 85 |