gilera dna lost power?

I have a gilera dna 50 and all was fine till the other morning when I was on the way to work and it just lost power and won't do more then 17 mph :( I have changed the drive belt but no joy! I noticed this morning tho when I started it up that when I rev it and its on its stand the rev counter will go up to 9 but when I take it off the stand and rev it then it won't got past 5. Please can some one help? I also noticed more smoke is coming out the exhaust then usual :(