Try
this to get the timing correct. Backfiring out of the carb means either the timing is too far advanced, or something is up with the valves or valve lash. If you've verified the crank/cam timing is correct, and you've verified the timing is correct with the above link I provided, then either the valve lash on the intake valve is too tight, or you didn't lube it enough and it's sticking open.
That brand new Walbro carb is non-adjustable and a POS, although that probably doesn't have much to do with this.