Two things cause backfiring through the carburetor, timing and a lean condition. If you didn't mess with the timing then carburetion is suspected. As you just rebuilt yours - probable.
Did you reconnect the accellerator pump lever; adjust it at all... unnecessarily? When the engine is off, can you see a clear double shot of gas spraying into the carb throats when you blip the throttle lever? If yes...
Did you adjust the idle mixtures too lean? Shouldn't have that big of an effect but if the set screw are still bottomed out it won't draw any gas through the venturies at tip-in. If 1.5 to 2.5 turns out each...
Did you set the float correctly? Too low will starve the engine after tip-in.
past rigs -