A little back ground on the bike. Bought the bike for my son to practice on as to not wear out his 250F. Bike had 64 hours on it and ran pretty good. We decided to put on a EG 134 kit, and this is where the problems begin. Never could really figure out the jetting on the bike, could never seem to get it crisp. With 17hrs on the kit it started to stumble and cut out as the RPM got higher. Ended up pulling the cylinder to find the exhaust bridge cracked. Sent it back, had it welded up and re-plated, new piston kit and $450 later we were ready to go. Did a couple of 5 min heat cycles, 1/2 throttle, up and down the road. did it one more time the next day. My son took it to the track, did a couple of laps and let it rip! He made about 3/4 of a lap and it locked up. Total run time was 15-20 minutes. This was about three weeks ago, temp was around 80, elevation 2800', 400 main, 40 pilot, stock needle- 2nd position, 2 out on air screw. The piston pictures below are what we found. The piston was fitted to the bore at Millenium. I called them up, explained what happened and they told me to box it up and send it back. I got a call from the receptionist, she said that everything checked out on their end but they would "good faith" warranty the cylinder and fix it for free. I had to buy the piston kit. I asked her what they thought would cause this and she said I had some sort of air leak. I asked to talk to Eric and was told that he was out of town. While the cylinder was away getting repaired, I decided to replace the crank, main bearings and all the seals. Got the cylinder back and put the bike back together. Everything went back together smoothly. Today is about 70, elevation 1350', 410 main, 40 pilot, stock needle- 3rd position, 2 1/4 out on the air screw. Again, did a couple 5 min,1/2 throttle heat cycles. Took it out for about a 10 minute easy ride and let it cool. It seamed to be rich on the low end, thought maybe I had gummed the plug during the heat cycles. Took it out again and rode it pretty hard, but didn't beat on it. Top end seams to be way lean again, but I am jetted per the manual. Did a couple short full throttle runs and both times half way through the bike just bogged and I had to let off the throttle to get it to come back on? The plug picture is from when I got back. Seems lean? If I have an air leak, I don't know where it would be. I don't want to seize it up again and I am hoping someone can give me some advise!