You need to check the clutch weights, they control your RPMs. It should be pretty easy to google the correct weights for your altitude.
If you check the spark plugs you can tell if it is running lean. They should be a nice dark tan color. If they are black and have soot on them, you are running rich. too light and you are running lean. You can look up a jetting chart for your altitude as well.
I would make sure you have a belt on there, and that nothing is broken with the secondary or the chain case/jack shaft ect.