Hello how are you
Ignition timing or cam?
If its blowing the carb off that tells us the engine is firing with the intake valve open. Hard starting is another sign of the spark plug firing way before top dead center.
I'll bet then the flywheel was put back on it was not properly aligned with the key in the crankshaft
Remove the side cover and look into the slot in the flywheel
If the flywheel key is sheared or not the proper size it will allow the flywheel to be off a couple degrees, that would mess your mark alignment up. The best way to check it is to remove the flywheel, my bet is you will find the issue with the alignment key
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