1. Confirm the drive clutch lever cable is fully moving the clutch control arm on the transmission and operating smoothly. Otherwise, the clutch inside will not engage and cause wear/limited drive power to the rear wheels. If using the original drive clutch lever, it may have stretched or is binding inside the sheath. Consider a new cable.
2. Confirm the tension spring on the transmission is fitted correctly, and the belt is snug around the input pulley and crankshaft. A loose / greasy belt will slip and also cause weak/no drive power.
3. Ensure the input pulley is properly fitted to the input shaft of the transmission with a roll pin fully inserted.
4. Make sure the rear wheels spin freely and smoothly with the clutch released. The adjusters and and pinion area must be clean and not binding, a small bit of general-purpose wheel bearing grease is needed, but don't overdo it.
5. Use a tachometer...move the throttle to FAST and engage the blade; engine must turn at 2,950~3,100 rpm to produce proper power/torque to drive the transmission.