I don't think it is carbs, and carb balance is not that big a deal unless it is
ridiculously off. Cams go flat though, not too noticeable at idle but really
when you rev them up. Usually (not sure why) it will be an exhaust lobe so that
after combustion the stuff doesn't have anywhere to go till the intake valve
opens, so «¡¡BANG!!» it comes back out the carb, slamming the carb's piston
down in the process.

I'd pull your valve cover and carefully measure the lift of each valve. It is
easiest to measure from the top head surface (where the valve cover gasket goes)
and the valve spring retainer). ALL of them should lift about 3/8". Since the
backfiring is in the back, start measuring at the back. About 20 years ago I
had the same deal with Li'l Red and after doing a valve job, changing cylinder
heads, changing carbs and a bunch more stuff finally noticed that if I pulled
#4 spark plug wire it stopped backfiring but would make a cloud in front of the
back carb (air cleaner off). Turned out that the exhaust lobe on #4 was pretty
much gone. I had tried to economize on lifters by regrinding them (all the way
through the hardened layer) and they retaliated by eating a cam lobe!
As IPD says, reusing old lifters = false economy.
--
George Downs, Bartlesville, Heart of the USA!