seems funny as it was running before. your timing shouldnt have changed too much.only thing i can think of is maybe it has a stuck open valve. if its firing incorrectly it would probably misbehave like back fire or something?
the first stroke the fuel valve opens to let in fuel. you said its getting fuel so that cant be stuck closed.the exhaust valve needs to stay closed till it fires then open on the exhaust stroke to let the the gases out. maybe the valves are broken or stuck open.
or the timing is 90 degrees out and its firing on the exhaust stroke.
