I know you say you have good spark. Even so I would check under the flywheel for rubbed wires. Very easy for this to happen unless you take great care to route the wires away from the moving flywheel. I have acquired a few that have brand new ignition damaged this way and then given up on . I repair with heat shrink and away they go.
The backfire through the carb usually indicates reversed wires and that too was my first guess.
