Inspect sprockets

Sprocket teeth wear as the chain rolls over them. A good tooth is the same shape on both sides with a rounded top. A worn tooth leans over like a hook or a wave, gets thin and sharp, and the chain can ride up and jump. The small front sprocket wears faster than the rear because each tooth carries the chain more often, but it is hidden under a cover, so riders often miss it.

Safety

• Engine off, key out. Turn the rear wheel by hand only.
• Sprocket teeth are sharp. Wear gloves if you touch them.

You need

• A torch and rags.
• Tools to remove the front sprocket cover (usually a few bolts).

Steps

1. Put the bike on the centre stand, in neutral. Wipe the rear sprocket clean.
2. Look at the rear sprocket teeth from the side. Compare the two sides of each tooth. Hooked, wave-shaped, thin or pointed teeth mean wear. K-Light 125: replace the sprocket when about a quarter of the tooth is worn away, or if it is bent.
3. Look for broken or chipped teeth. Any broken tooth means do not ride until replaced.
4. Look at the face of the teeth. Shiny wear on only one side means the chain runs at an angle: the wheel is not straight or the sprocket is bent (Check and adjust chain slack).
5. Turn the wheel slowly and watch the sprocket edge against a fixed point. A wobble means a bent sprocket or loose nuts.
6. Check the rear sprocket nuts are tight. Hold the sprocket (with gloves) and rock the wheel: a small springy movement is the cush drive rubbers; a large free movement means they are worn (Check the cush drive rubbers).
7. Remove the front sprocket cover. Clean out packed dirt and old grease. Look at the front sprocket teeth the same way.
8. Check the front sprocket bolt or circlip and plate are in place and tight. The sprocket must not slide on its shaft.
9. Look behind the front sprocket for oil. Oil there means a leaking output shaft seal: Oil leaking behind the front sprocket.
10. Refit the cover.

Decide

• Teeth good and chain good: keep riding and keep the chain lubricated.
• Either sprocket worn, bent or broken: replace both sprockets and the chain together. The K-Light manual warns that mixing new and worn parts makes the new part wear out fast. See Replace the chain and sprockets.

Check your work

• You have looked at both sprockets, all the way round, and checked they are tight.

If it did not fix the problem

• Sprockets good but the chain still jumps: Chain jumps teeth or comes off the sprockets.
• Sprockets wearing on one side: Chain rubs on the guard, slider or swingarm.

Reference values (K-Light 125)