I am having a problem with my timing chain falling off the bottom sprocket. I have put it back on once and it fell off again almost immediately. I have checked the spring on the chain tensioner and it seems ok.

I have recently installed a big bore kit and when reinstalling I came up short on one of the bolts on top of the head at the cam, I know this was stupid but I ran it without it, my dealer was taking forever to get the bolt in and I'm still not sure if it is in yet. I really don't think it is the cause of the problem being it's coming off on the bottom sprocket but I wanted to include everything I know. The cam also has a tiny bit of play in it but not much.

I am pretty famialiar with the top end but am almost clueless to the bottom. I'm sure if I know what the problem is I can get in there and fix it though. I know I can take it to a dealer and get it fixed but I would rather do it myself so ANY information would be GREATLY appreciated. Thanks for your replies.

 

Did you replace your cam chain when you put the big bore kit on? Your chain may be stretched out more than what the tensioner can tension.

Are you sure the cam chain tensioner guide is in correctly? The bottom of the guide fits into a groove, maybe you missed it? You can take a small screwdriver and turn the screw inside the tensioner with the engine running, and you should be able to feel/hear the tensioner guide putting stress on the cam chain. After you do this, you will need to pull the tensioner off and reset it to its normal spring tension.

Every so often I run into a 4 stroke that the cam chain sprockets are worn out. The bottom sprocket is a press on/off. If you do it yourself, there is no keyway or location identifier, so use the keyway on the end of the shaft to identify if it should be in line with the tip or valley of the sprocket teeth.

I'll be putting a cam chain in a 250X in the next couple of days, so if you are still having problems, the internals will be fresh on my mind.