i recently rebuilt my upper end on my 250x. after 5 or 6 hours of riding my chain slipped off at the drive gear in the crankcase. when i loaded the chain tensioner i turned the slotted screw to retract the spring then i bolted it to the block and then released the the slotted screw the chain was tight and all seemed well. what could cause the chain to slip off the bottom gear. the obvious answer is the chain was loose but i do not understand why. any ideas?
thanks dan?

 

The cam chain guide towards the front of the engine fits in a slot right by the lower gear. Make sure the end of the guide is in the slot. If it's not your chain will not have proper tension on it and it will do what you describe. Everything might look fine and dandy from up top but for the cost of an $8 clutch cover gasket you will know for sure if the guide is in the slot. Your dealer should have a gasket in stock.