As you open the throttle from a stop, the scooter shakes or shudders for a moment before it pulls smoothly. The judder comes from the clutch shoes gripping and slipping in quick turns against the clutch drum (bell). Common causes are a glazed, grooved or out-of-round bell, oil or belt dust on the shoes, worn or uneven shoes and springs, and worn rollers. On many scooters the engine and drive swing on rubber mounts, so worn engine mounts make judder worse. For limits and settings, follow your scooter's manual.
Is it safe to ride?
Ride with care. Judder makes slow manoeuvres and hill starts harder to control, especially with a passenger.
Quick checks first
- Does the engine idle smoothly?
- Is the idle speed higher than usual?
- Does it judder only after the scooter got wet, or after a service?
- Look under the scooter at the engine mount bushes for splits.
Diagnose it step by step
1Does the engine idle rough or misfire?
- Yes: an uneven engine causes uneven take-off. Go to Rough, lumpy or uneven idle.
- No: go to step 2.
2Is the idle speed high, so the clutch starts to grip as soon as you touch the throttle?
- Yes: Idle speed is too high; on carburettor scooters Adjust carburettor idle speed and mixture.
- No: go to step 3.
3Are the engine mount bushes (where the engine swings on the frame) or the rear shock bushes split, worn or loose?
- Yes: Check engine mounting bolts, Check the rear shock absorbers.
- No: go to step 4.
4Open the drive case (engine cold). Is there oil, grease or a lot of belt dust on the bell and clutch?
- Oil or grease: find the leak (Oil leaking from the scooter rear gear case). Contaminated shoes usually need replacing.
- Heavy dust: clean it out with brake cleaner and check the belt (Inspect and replace the scooter drive belt).
- Clean: go to step 5.
5Are the variator rollers flat-spotted or worn unevenly?
- Yes: Check variator rollers and sliders.
- No: go to step 6.
6Still judders?
- The bell may be glazed, grooved or out of round, or the shoes and springs worn. Go to Check the scooter clutch and bell.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| High idle | Clutch bites at idle | Idle speed is too high |
| Worn engine or shock mounts | Split or loose bushes | Check engine mounting bolts |
| Oil or dust on clutch | Seen with case open | Fix leak, clean; Check the scooter clutch and bell |
| Worn rollers | Flat spots | Check variator rollers and sliders |
| Glazed or grooved bell, worn shoes | Workshop inspection | Check the scooter clutch and bell |