I'm having an issue with my '77 860GT. It's running/starting well but when I close the throttle down to idle from around 4000 rpm or so, many small backfires occur. Not the big fire blazing, muffler expanding explosions that I thought were really cool when was 16 in my Malibu, but loud enough to know something is not right.

The timing is right on and I sync'd the carbs with mercury gauge. Idle is smooth and engine runs and accelerates smoothly out of idle.

Could this be a jetting issue or an idle mixture thing? I haven't looked at what jets I have in there.

The bike has relatively open aftermarket mufflers and no crossover pipe, but other bikes a had with these mufflers and stock jetting did not show this backfiring.