Crank gear or Cam Chain Tensioner Problem?

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.

check the guides to make sure they are all ok. Could possibly be worn form stretched chain. Did you check cam chain tension when you took the cover off after you had the problem?

Cam chain was extremely loose. In fact when I got the new chain I laid it next to the old chain and they both had 114 rivets, but the new one was visibly much shorter than the old one. I could lift it up about 3/4" before removing the tensioner. After replacing it with the new one it was tight as I would expect it to be with like 1mm of deflection with the tensioner installed.

I am going to pull the rotor tomorrow night(I have duty tonight, I am in the Navy) and look at the crankshaft gear teeth to see if they are extremely worn. I replace both camshafts with HotCams so those cam gears are brand new. I am really hoping the tensioner is bad(though it feels fine) because I really don't want to do the bottom end.

Pulled the rotor tonight after work and reinstalled the cam chain tensioner. It gets tight and everything looks good with it. I guess I failed to notice on the original cam chain replacement that it had gotten so loose that the cam chain retainer(stamped steel deal that holds cam chain against bottom of crank gear) had gotten kind screwed up and bent up. Here's a picture. Just ordered a new one from Motosport(they had it in stock, I'll have it Tuesday or Wednesday). The crank gear looks fine and the cam chain tensioner seems to work. I may install a Tokyo Mods Manual Tensioner anyway later. I will let you know next week if this keeps the cam chain from jumping the gears after my next ride.

Got new cam chain retainer part in and installed it tonight. I think I found my smoking gun. After installing the part I could tell that the new part definitely holds the cam chain on the crankshaft gear much tighter than the old chewed up one. I am going riding tomorrow to see if I have the same problems of slipping cam chain causing timing to go out of whack. I'll let you guys know if it is fixed.

Broke the bike in for two hours, changed oil and then got after it. In technical terms, it runs like a striped ape now. I was swapping back and forth between my buddy's CRF450 and my now RMZ290 with mildly ported head and Hotcams and we both noted that when full throttle on my RMZ290 it the pretty much the exact spot on the CRF450 throttle that you have to stop at before full throttle so you don't feed it too much. We are both B-class Hare Scramble racers so if you are faster or do western desert stuff this may not apply, but for eastern woods racing I think this is a wonderful combination. The Race Tech gold valve in the front end with 0.46kg/mm springs in this bike made me reach for a sixth gear(that of course didn't exist) in the whoops. Swapping back to the CRF450 and following my buddy through the same whoops I felt the shock of each whoop at the same speed. I think we will be revalving his bike soon. Anyhow no cam chain slippage. My recommendation is to pull the flywheel and check this stamped steel piece every time you change the cam chain which is a high wear item.

Hey thanks for keeping the thread updated man. i recently had the chain slip on my bike, i will definitely have to pay attention to that piece when i replace the chain

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