Engine revs, bike barely moves — classic clutch / drive engagement failure. Platform matters. These are chain-drive pedal mopeds with centrifugal or ramped clutches — not scooter CVT roller advice.
Puch E50 (single-speed centrifugal)
From the Puch Maxi guide: shoes in the bell grab by RPM. The left lever disengages for pedal start. Glazed bell, weak springs, or oil on shoes = slip. Tune levers on this platform: shoe weight, springs, jetting, pipe — not a scooter variator.
Puch ZA50 / Tomos (2-speed automatic)
Internal centrifugal clutches. From the Tomos guide: slipping, hanging in first, or slow shifts often trace to worn clutch pucks, wrong gearbox ATF (Type A / Dexron or Type F), or shim issues. Gearbox oil is separate from premix. A55 capacity note in that guide: about 10 fl oz (300 cc) if the level screw is inaccessible — still verify for your cases.
Honda Express / Hobbit
Ramped clutch / variator system (not an E50 bell). Symptoms from the Express guide: low top speed with high RPM (glazed pads / worn ramp), shudder on takeoff (weights / contaminated transmission oil). Use the correct transmission oil weight — wrong oil changes engagement RPM.
What to do under load
If the clutch is slipping under load on a hill, stop frying it — fix engagement before you buy a top end. Confirm oil approach first on Hondas (oil pump). Chain slap on Tomos is a different symptom: Tomos chain slap.