My son's kx85 quit running on us this past weekend and I am a bit confused at this point. When kicking the over the motor, a backfire comes out the exhaust? The bike has good compression, I've removed the exhaust and head to find perfect wear conditions on both the piston and cylinder.

It appears that the bike may have overheated due to the low amount of coolant that drained out of the system during the tear down. Also, the air cleaner is completely dirty. Could a dirty air cleaner be the cause of not running or would that just allow poor performance?

I am also concerned that the KIPS power valve assembly could be malfunctioning. It is currently wide open?