i would try cheaking the timing and a valve clearances. my yamaha was doing that and when i rebuilt the top end and set the clearances, it ran great after, well except it burnt oil, but thats a totally diffrent story.
84'200s (Dad bought new in 84, long gone)
84'YTM200 (sold 02)
85'KLT160 (sold 05)
84' Big Red (Cobra exhaust,good shape) sold in 08
84' Big Red (Parts Bike, runs good though)sold in 08
85' 250SX (New plastic, Clark Plastic Gas Tank, Web Cam Camshaft, Cobra Exhaust, Moose Jet Kit, etc..) sold in 2010 and highly regret!!!
04 Polaris Sportsman (Swapped 400 motor with a 500, Polaris wheels w/Kendra Bearclaws, Moose Fender Flares, Brushguard, Warn Winch)
84 200s (Needs work!)