Each time you open the throttle after closing it, or close it after accelerating, you feel a jolt and hear a clunk from the back of the bike. This is slack in the drive line being taken up suddenly. The cush drive (rubber blocks between the sprocket carrier and the wheel hub) is meant to soften this; when the rubbers go hard or wear, the jolt comes through. A very loose chain, loose engine mounts, a worn swingarm pivot or worn wheel bearings make the same kind of clunk.
Is it safe to ride?
Ride with care. The clunk itself is not dangerous, but it wears the chain, sprockets and gearbox faster, and some of its causes (loose mounts, swingarm play, bearing play) affect handling.
Quick checks first
- Check chain slack.
- Bike on the stand, in neutral: hold the rear sprocket (gloves) and rock the wheel back and forth. How much free movement is there?
- Grab the rear wheel at the top and bottom and try to rock it sideways.
Diagnose it step by step
1Is the chain slack above the specified range? (K-Light 125: 20 to 40 mm, bike upright.)
- Yes: Check and adjust chain slack, then test again.
- No: go to step 2.
2Holding the sprocket, is there a clear free gap when you rock the wheel, before the rubbers push back?
- Yes: the cush rubbers are worn. Check the cush drive rubbers.
- No: go to step 3.
3Does the rear wheel rock sideways on its axle?
- Yes: Check wheel bearings; see Wheel moves sideways on the axle.
- No: go to step 4.
4Grip the end of the swingarm and try to move it side to side. Is there play?
- Yes: Check the swingarm pivot.
- No: go to step 5.
5Are the engine mounting bolts tight, and the shock absorber mounts tight with good bushes?
- No: Check engine mounting bolts, Check the rear shock absorbers.
- Yes: go to step 6.
6Is the clunk from the engine side rather than the wheel?
- A clunk from inside the engine on throttle changes can come from worn clutch basket slots or gearbox dog play. See Clutch grabs or judders for clutch checks. A rattle from the exhaust: Rattling from the exhaust or heat shield.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Loose chain | Slack above range | Check and adjust chain slack |
| Worn cush rubbers | Free gap rocking against the sprocket | Check the cush drive rubbers |
| Worn wheel bearings | Wheel rocks sideways | Check wheel bearings |
| Swingarm pivot play | Swingarm moves sideways | Check the swingarm pivot |
| Loose engine or shock mounts | Bolts loose, bushes split | Check engine mounting bolts |
Reference values (K-Light 125)
| Item | Value | Source |
|---|---|---|
| Chain slack, bike upright | 20 to 40 mm | K-Light 125 manual, PDF p. 61 |