i found this post on dirtrider.com and thought it might be useful for those who haven't heard about this problem before. jimt_yz400 "I installed a new piston and rings into my 250F. It really didn't need it after measuring the ring cap of the old gap but I did it anyways since I planned on racing the D36 Enduro series in NorCal. This went well and I rode the bike for few miles on Sat. But there was a pblm. The bike would idle at 2000 or more rpm and would not always fall. I thought that it was jetting of and air leak but it didn't act like it had an air leak of lean on the bottom. The bike ran great all over but it would idle really high. I found out later that day the I had the exhaust and intake cams one tooth advanced. Ok, so last night I adjusted them and set them the way that manual states. The next it would not start. I checked tht plug and it was fine. I then took it to the track and a friend of mine took the valve cover off and we checked the cam timing and I adjusted the fule screw a little and it then fired right up, idled perfect and had good throttle response. My son took it out to the track and it died on him before he could get there. I gave him my bike to ride and jumped on the pblm bike. I started if and it it idled fine but then died and made a strange "clank" like a chain being yanked after it died. I then pumped it through a few times and it seemed to be ok, no weird sounds etc. though it did feel a little stiff going thru the stroke when I was attempting to kick start it. It then started right up and I rode a few laps (two) then it started to have some low end response pblm like low fuel/low float bowl level. I took it back and shut it off. It then was very difficult to kick over though not like a piston seize. Ok, I tore it apart and here is what I found: Intake cam was difficult to rotate, the exhaust was ok. I removed the intake cap and cam and the journal nearest the clutch side was scoured as well as the cam and the cap. There was also some scoring on the other journal on the intake but not as bad. The exhaust was scored very little but it's not perfect." The response by the other members showed that this is a problem, and one member stated he had seen it many times before. Due to the very high tolerences in the engine, the cam caps require a very small torque wrench setting. Im told that WRs have a different setting to the YZ. YZ=7.5ft/lbs WR=7.2ft/lbs. From what i have learned, this can happen if the camshaft caps are tightened too much due to human or torque wrench error (doesn't have to be by much. No more of the 'and a bit more for good luck'), or if the tightening proccedure isn't followed correctly (as stated in the manual). It is also a good idea to lubricate the camshaft caps before reassembly.