The bike runs well when dry, but in heavy rain, through deep puddles or after washing it misfires, loses power or stops. Water gives the high-voltage spark an easier path to earth than the plug gap. It also gets into connectors and switches, and can enter the airbox or the fuel tank. The ignition system is the most likely place, because the plug lead and cap carry many thousands of volts.
Is it safe to ride?
With care. Ride slowly through standing water and avoid deep puddles until it is fixed. If the engine stops, move off the road before checking.
Quick checks first
- Is the plug cap pushed fully on, with its rubber seal in place?
- Is the plug lead cracked, or the coil body cracked?
- Is the kill switch housing cracked or full of water?
- Does the airbox have water in it, or is its drain blocked?
- Has the fuel cap seal failed (water in fuel)?
Diagnose it step by step
1In the dark, with the engine idling and the plug lead and coil sprayed with a fine water mist, do you see small blue sparks on the lead, cap or coil?
- Yes: high voltage is leaking. Replace the cracked cap, lead or coil. Test the ignition coil and plug cap. Keep well clear of the lead while the engine runs.
- No: go to step 2.
2Is the outside of the spark plug insulator dirty, cracked or wet, or is the cap seal missing?
- Yes: clean or replace the plug and refit the cap firmly: Spark plug service.
- No: go to step 3.
3Does the engine stutter when you tap the kill switch or side stand switch while wet?
- Kill switch: Test the engine stop switch.
- Side stand switch: Test the side stand, clutch and neutral switches.
- Neither: go to step 4.
4Open the main connectors near the coil, pick-up, ECU or CDI. Is there water or green corrosion inside?
- Yes: dry and clean them, and seal against water: Clean and seal electrical connectors. The K-Light 125 manual notes most ignition faults come from poor connectors. See Electrical faults after rain or washing.
- No: go to step 5.
5Is there water in the airbox or a wet air filter?
- Yes: clear the airbox drain and dry or replace the filter: Clean or replace the air filter. Check the airbox lid seal and the intake snorkel are not facing the spray from the front wheel.
- No: go to step 6.
6Drain a fuel sample into a clear jar. Is there water at the bottom?
- Yes: Drain contaminated fuel and service the fuel cap seal.
- No: do a spark test while wet, and see Weak, thin or orange spark.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Cracked plug cap, lead or coil | Sparks visible on mist test | Coil and cap test |
| Dirty or wet plug insulator, missing cap seal | Visual check | Spark plug service |
| Water in kill switch or stand switch | Stutters when tapped | Kill switch test |
| Water or corrosion in connectors | Open and inspect | Connector clean |
| Water in airbox | Wet filter, blocked drain | Air filter |
| Water in fuel | Layer in jar | Drain fuel |