Top end screaming with heavy rollers help me please !

My piaggio zip has got 6.5 gram rollers and a Mollossi belt but when I ride it it gets to a certain point were it just screams it's head of but there's no power there like I'll be doing 40mph then will start screaming and won't go any faster just revs higher it's really annoying and I need to no why if anyone can help me thanks

 

Your drive belt is worn out and slipping I reckon. You probably need a new belt.

 

or you have some other transmission issue. Remove the cover and have a look !