As stated, the centrifugal clutch lets you come to a stop with the motor running and the trans in gear. Without it, you would stall the motor when you came to a stop. It can be hard to diagnose slipping problems between the two. The best way is disassembly and measurement of the shoes and comparing it to the manual. However, I observed a problem on my centrifugal clutch that may be part of your problem too. Put the machine in gear and let it sit and idle. Does the machine want to creep forward? Do you have to hold the brakes to keep it back? In my case, there is a link (just like a chain master link) that the centrifugal clutch shoes pivot on. The hole in the clutch shoes was severely egged out. This let the pivot point of the shoes engaged the hub and grab. If you have this symptom, you'll need to replace your centrifugal clutch shoes.
Other problems that you can identify based on a teardown are:
1. Warped steels in the shifting clutch. Warped steels won't let the clutch disengage properly when shifting
2. Cooked or worn out fibers on the shifting clutch. Will cause slipping when trying to accelerate
3. Cooked or worn out centrifugal shoes. Will also cause slipping when accelerating.
4. Siezed one-way bearing - when you shut down the motor, if this bearing is seized, you'll hear a spinning, whirring noise after the engine is stopped.
Given your symptoms, I'd start with the centrifugal clutch. Sounds worn to me.
- Frank
1984 200ES Big Red
1985 350X (x2)
1986 350X
1986 250SX
1984 Auto-X
1984 ATC70
1985 ATC70