If it idles better with the choke out, that means its lean. You have to figure out if you have an airleak, or if The carb is set up properly for you mods.
With your setup, a stock carb would be best. Make sure its cleaned well, Floats set 20-21.5mm, 32.5 pilot jet, the air screw should be set 1.5 turns out from lightly seated, and a main jet should be 280-300 range.
Jet kits never have the jets you need, and are way over priced, You can buy those individual ones for probably under $20.
Backfiring is sometimes a caused by a bad set of reeds, or a flywheel that has sheered a key that's not letting it fire at the right time.