by GregR » 18 Sun Jul, 2010 5:44 pm
Is it backfiring through the carbs or the exhaust?
In either case, I'd check for an intake or exhaust manifold leak, possible lean carb(s), one carb opening before the other (carbs out of sync), to name a few.
Did you replace gaskets when you replaced the carbs? Make sure everything is tight.
Since it sounds like this has been an on-going problem, just a bit worse, are you valves set to 0? Not a jammed tight zero, but not loose either. That's how mine are set.
GregR