[silent one]

Have an 04 650 arctic cat. When new it would run around 75-80 now only 50-60. Have change belt and a few other odds and ends to try and get it back to normal. It seemed to slow down pretty drastically. Some one told the valves needed adjusting, and it has had tune-ups since. Does any one have any idea what may have caused it to slow so much? or is it normal for a belt drive to slow this drastically? Thanks for any possible help..