Could be one of several hundred possibilities. Sucked something into the middle carb and it digested through the middle cylinder, Broke a reed and it closed the plug gap, broke a ring and it closed the gap, the keeper may have fallen out of the piston and the ring turned causing an end to snag a port, the list goes on and on. Only way to tell is to pull the head and start an autopsy. If it closed the gap on the plug, it ain't gonna be cheap. It's just to see how much damage to which parts. Could need a re-sleeve, could be a simple bore job.
D
|