Scooter CVT After Home Tuning and a Can of WD-40
A 110 cc automatic scooter with 21,000 km arrives with poor acceleration and a top speed that has fallen from 90 km/h to 72 km/h. The owner recently fitted 12 g aftermarket rollers in place of the stock 9 g, and last week sprayed WD-40 into the CVT case to stop a squeak. The engine now flares when pulling away but the scooter barely moves, and after 10 km there is a burning rubber smell. Measured: belt width 21.4 mm (new 23.0 mm, service limit 22.0 mm) with glazed edges; drive face and boss show no deep grooves; all six rollers weigh 12.0 g with flat spots on three; clutch shoe lining 2.1 mm (limit 2.0 mm); clutch bell blued with a mirror glaze; contra spring free length 87 mm (new 95 mm, limit 91 mm). Variator sliders are worn but intact. The owner says just change the belt, the rest is fine, and he needs the scooter for a 60 km delivery run tomorrow.
1. Rank the faults by their contribution and explain the symptom chain.
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- The belt at 21.4 mm is below the 22.0 mm limit. A narrow belt sits too low between the variator faces at take-off, so the starting ratio is wrong, the engine flares and the scooter creeps. At speed it cannot climb to the outer edge of the drive face, which is exactly why top speed fell from 90 to 72 km/h.
- WD-40 on the clutch shoes and bell. The lining is glazed, the bell is blued, and the burning smell after 10 km is clutch slip, not just the belt.
- The contra spring at 87 mm against a 91 mm limit lets the belt ride too high too early and adds slip under load.
- The 12 g rollers change the shift point and three are flat-spotted, but heavier rollers alone would lower engine speed for a given road speed, not raise it. They are secondary. So the high rpm is slip, not gearing, and the diagnosis should be presented that way.
2. Which parts do you replace and which do you keep? Justify each with the measurements.
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Replace the belt: 21.4 mm is past the 22.0 mm limit, so there is no judgment call. Replace the clutch shoes: the lining is glazed and contaminated and the bell is blued, and solvent-soaked friction lining cannot be cleaned back to specification. Replace the contra spring: 87 mm is below the 91 mm limit. Replace all six rollers, since three are flat-spotted and the set should stay balanced, and recommend returning to stock 9 g unless the owner has a specific tuning goal. Keep the variator drive face and boss, which show no grooves. Keep the sliders, worn but serviceable, and note them on the next-service line. Before assembly, wash all WD-40 residue out of the CVT case. Any oil left inside will re-contaminate the new shoes. Also check the final gear oil while the cover is off, and measure the clutch bell inner diameter before deciding to replace it.
3. The owner says just the belt. How do you handle it, and what goes on the job order?
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Sell the reasoning, not the parts list: a new belt against a slipping, oil-contaminated clutch will glaze or burn within a few hundred kilometres, and the burning smell he already noticed is the clutch. Offer a clear choice: belt only, with no warranty on driveability and a written note that the clutch and spring are already past limit, or the full set with a workmanship warranty. Say it once, calmly, and let him decide. The 60 km delivery run tomorrow is the real constraint. Tell him plainly that under load the slip and heat get worse and the scooter may not finish the run. Job order: measured belt width, spring free length, lining thickness and bell condition against limits, photographs of the glazed bell, his signature on the chosen option and initials on the declined items, and a note about the WD-40, since it caused the clutch damage and is relevant to any later warranty claim.