I think you put the sleeve or piston in wrong when you rebuilt it. By doing that, you made the ports deliver fuel in the wrong order, making the fuel detonate/explode too early, which makes the piston go the wrong way.

When it warms up, the timing must change just enough for it to fire somehwat correctly.

The bang is probably SEVERE detonation when the piston slams into the other direction. Makes sense.


You are destroying your engine....and the rod might be shot -- maybe the piston, too.

Reconfigure the sleeve AND the piston, because they are in the wrong way. The sleeve ports and piston skirt must be in the right position.