Just my .02 worth....
Backfires don't occur because the carb is lean (or fat either for that matter). Backfires are timing related where the intake valve is partially open when you have a charged cylinder lighting off. If the intake valve is closed - then there could be no "back fire" thru the carb.
Lean conditions usually present themselves as "misses" -- bucking - or pinging/knocking... surging.
I would be checking your spark plug wires... to make sure you have them right on the cap and the plug they go to. Check your timing - and that doesn't mean just checking the timing setting with a light - it means to make sure that your timing pointer and balancer are actually at zero when at TDC and that the motor is physically at TDC when they say that's where you are.
The other thing that can cause you're condition is a valve that is sticking and or a valve lash that is keeping the valve from seating.
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