Replaced piston rod. Engine was backfiring after put back together. Took engine apart and rotated crank one turn till marks on cam and crank lined up again. Still backfireing when attempted to start. Has great spark. Seems to be exhausting back through carb when attempted to start both times (thats why I tore engine apart 2nd time, thought time was 180 off)

Did you reset your valve clearance,
Make sure the flywheel key hasn't shear . This will happen sometimes when they backfire.
Luck