2007 RMZ250 had a worn out cam chain(way stretched out). I pulled top end apart and discovered piston undersized and cylinder needed replating. I put in an Athena 290cc kit with a new camchain in, cleaned up the head and removed the casting flash(not really porting just cleaning up to smooth stuff out), clearanced all the valves, and put in a new Barnett Clutch basket because the old one was toast. Switched to a 42 pilot jet from a 45 and 1.75 turns out on the air/fuel ratio screw and she started right up and I rode her around the neighborhood to get a heat cycle in her(0.3 hours on my hour meter) at barely cracked throttle and short shifting her to allow the rings to seat. I took the bike out the next day with my family to break it in and after 0.4 more hours for another heat cycle I went to refire the engine and the kickstarter would stick at TDC and would not allow me to kick it over. I pulled it apart and discovered a decompressor on the exhaust cam was hanging up and operating sticky. I took it apart and lapped it where it mates up to the cam gear. Now all is well bike fires right up no more issue with compression locking at TDC. So I go out again with my family the next day. After 1.1 hours of riding on my hourmeter at moderate throttle openings and not revving it out I get a really bad rhythmic clacking and I again shut the motor down and push back to the truck. I watched and listened to the video on my GOPRO and the sound is sickening. I pull the motor back apart and immediately discover that the cams are not lining up at TDC(i.e. the camchain has slipped). I can roll the motor over with the crank nut and I feel no valves hitting the piston(whew!). I did NOT replace the automatic cam chain tensioner because the original one still seems to work fine(i.e. it extends when out of the bike when I put the spring/rod in with the center bolt). What is causing my cam chain to slip on the gears? Cam gears look good to me. Worn crank gear or bad tensioner are my thoughts what do you guys think? I really don't want to do the lower end too if I don't have to.