hello everyone. I wanted to post a recent experience so others that might run into the same problem won't pull too much of there hair out trying to figure out the solution. I have an 1981 xt250. I replaced the stock airfilter and box with a unifilter, put a megacycle cam in it with r and d valve springs. New exhaust valve+ valve job, and bored and honed for a 0.5 piston (found a good amount of corrosion on the cylinder wall). After all that buttoned up the engine and it fired right up. It was running a little lean so I ordered some new main jets to tune it up. After finding the right set up it ran great for about a day. The motor started to hesitate and missfire at 3/4 to full throttle really bad. I could hit the redline if I stayed under that throttle position, but as soon as I opened it up it had nothin.