CVT Judder and Lost Top Speed on a Two-Up City Scooter
A 110 cc automatic scooter has covered 21,500 km, mostly two-up in city traffic, and its CVT cover has never been off. The owner reports a violent judder when pulling away between 2,500 and 3,500 rpm, a squeal during hard acceleration, and top speed down from 85 km/h to 72 km/h. On road test the engine speed rises before the scooter moves, and the judder disappears above 20 km/h. Strip-down findings: drive belt width 16.1 mm with frayed edges (new 18.5 mm, service limit 17.5 mm); roller weights with visible flat spots, measured across the flat 17.1 mm against 18.0 mm new and a 17.6 mm limit; clutch shoe lining 1.4 mm against a 2.0 mm limit; the clutch drum inner face is blued and glazed but not scored; the contra-spring free length is 1.5 mm below the service limit; final drive oil is grey and smells burnt. The customer asks for just the belt because a complete kit is expensive.
1. Explain how the worn belt, flat-spotted rollers, glazed shoes and weak spring together produce the judder, the squeal and the loss of top speed.
Read the worked answer
The CVT sets ratio by how deep the belt sits in the front variator and how far out it rides at the rear clutch pulley. Each worn part shifts that balance.
- Belt at 16.1 mm instead of 18.5 mm sits deeper in both V grooves. At the front it cannot reach the small radius needed for a low ratio, and at the rear it cannot climb to the largest radius, so the highest ratio is lost. That is the 85 → 72 km/h top speed.
- Flat-spotted, undersize rollers make the variator move in steps instead of smoothly, so ratio change at the engagement point is jerky.
- Clutch shoes at 1.4 mm with a glazed drum grab, slip, grab, slip - stick-slip at low speed. That is the judder, which ends once the clutch is fully engaged above about 20 km/h.
- Weak contra-spring cannot hold belt tension, so the belt slips under acceleration. That is the squeal and the engine revving before the scooter moves.
- Grey, burnt final-drive oil shows the final drive has been running hot from the slipping; check the drain plug for metal.
Each symptom points at a different worn part, which is why one new belt cannot cure all three.
2. The customer wants only the belt. Decide what must be replaced and what can be reused, and explain the consequence of fitting the belt alone.
Read the worked answer
Replace
- Belt: 16.1 mm against a 17.5 mm limit, with frayed edges - mandatory.
- Rollers and sliders: flat-spotted and below the 17.6 mm limit - mandatory and inexpensive.
- Clutch shoes or complete clutch assembly: 1.4 mm against 2.0 mm - mandatory; with a blued drum, a complete assembly is usually better value.
- Contra-spring: 1.5 mm below free length - replace.
- Final-drive oil: replace and inspect the drain plug magnet.
Reuse after inspection
The clutch drum only if the inner diameter is within limit and the face is not scored. Deglaze with 320-grit in a circular direction, wash with brake cleaner and dry. Check the variator boss and ramp for steps or ridges; replace if ridged. No grease or oil on any sheave face.
If only the belt is fitted
The new belt runs against flat-spotted rollers and a weak spring, so it cannot reach the correct ratio. The glazed shoes still judder, and the customer returns within a few hundred kilometres with a belt already contaminated by dust from the worn parts - and pays the labour twice. Explain this with the measurements rather than as an upsell.
3. State the reassembly checks and the road-test and follow-up plan.
Read the worked answer
Reassembly
- Belt direction arrows correct and the belt seated fully in the rear pulley by squeezing the shoes.
- Variator nut and clutch nut torqued to specification using a holding tool, not a rattle gun on the crankshaft.
- All sheave faces degreased; cover gasket in place; CVT duct and filter clean.
- Final-drive oil filled to the correct level with the specified grade.
Road test
Two-up pull-away from standstill; no judder between 2,500 and 3,500 rpm; no squeal; acceleration and top speed close to 85 km/h; cover not overheating after the run.
Follow-up and records
Re-check at 500 km. Advise CVT cleaning every 8,000–10,000 km for two-up city use, since this unit was never opened in 21,500 km. Record the measured belt width, roller diameter, shoe thickness, spring free length, parts replaced, torque values and the road-test result on the job card.