Yep, sounds like the centrifugal clutch is slipping, but it could be the change clutch so inspect those too once you've got the cover off. You may have to replace the drum along with the weights and springs, its probably grooved by now. You won't know for sure until its apart.
For now, try to readjust the change clutch to see if that improves anything. Also make sure that the motor oil you've been using is JASO MA or MA2 approved for wet clutches.
For now, try to readjust the change clutch to see if that improves anything. Also make sure that the motor oil you've been using is JASO MA or MA2 approved for wet clutches.