My BJ8 had this problem from the day I bought it, what I discovered was the one of the needles was not properly located in the piston. The old style needle has no shoulder on it to indicate proper placement in the piston, this needle doesn't even have the groove that some needles do. What I discovered was that one carb would be RICH at idle, and quite lean at normal operating speeds while the other carb was within limits at both speeds. One might think a different needle was required. What I discovered was that by lowering the sholderless needle, I could get the same operating balance as the other carb.
I purchased a Gunson Gastester to discover this problem. Since reseating the needle and adjusting the CO level to about 2.75% at idle, the car has become a dream to start, I can remove the choke sooner in the warm-up cycle and seldom (very cold days choke off too soon) do I have that nasty backfire.
The car came with numerous other little carburetter features that have also been sorted, like the wrong springs, and idle speed adjustment via use of the Fast Idle adjusting screws. These were very easy to find compared to the mis-aligned meetering needle.
Hope you have luck in finding you gremlin.