Have a Moto4 that suddenly lost compression mid-ride. The bolt on the timing chain sprocket backed off enough that I could turn it by hand, so i imagine that why it broke the guide pin in the camshaft. Camshaft was ruined so i replaced it an the sprocket. Got the engine set to TDC, an got the camshaft an sprocket installed so the line on the sprocket lined up with the indicator on the cylinder head. Before replacing anything, i could turn the engine over by hand with no resistance, now i can definitely feel the compression but still dosnt feel quite right. Hard to explain, but just dosnt feel like all the compression is there an like it’s not getting any fire.

So my question is, could something else might’ve been affected in the process or could’ve something else caused it’s issues to begin with? Any ideas of what else to look at? Thanks

Also a side question, the electric start hasn’t worked right for a long time. It would turn over an start with the motor, but would kill the engine once running, like it wasn’t disengaging completely. Maybe the starter clutch not disengaging? How much of a pain is it to fix?

 

I would guess the 250 Yam engine is an "interference" type, that is, if the drive to the cam fails, the valves hit the piston. You will probably have bent a valve, which is now not seating properly, dropping compression. Do a compression test to find out. If you do need the head off, check the cam chain. I have just done a Honda 250 Recon which broke its cam chain adjuster and the timing slipped, the chain was very stiff, no idea why, though Honda chains are invertooth. It bent a valve too.

Regarding the starter clutch, remove the starter and turn the gear it drives round with a screwdriver.It should turn easily in one direction but moves the crank round if you turn it the other way.If it moves the crank in both directions, the clutch has seized, requires the flywheel removing to get at it.