One cause of popping/mild backfiring through the exhaust system can be the pulse air injection system
(the plumbing inside the airbox and the pipes&hoses connecting from the airbox to the cylinder heads under
the exhaust nuts). The vacuum lines are supposed to shut off the air injection to the headers when the throttle
is closed, but it doesn't always work perfectly well. If your carbs are running a bit on the rich side this can happen
as well as some unburnt combustibles do make it through into the exhaust system.
On my LS, I can get some light pops/crackles if I run it up quick to around 6000 RPMs and quickly chop (let go) the throttle.
I still have the pulse air system and I am intentionally running the mixture just a little bit richer than absolutely
necessary to help keep the head temperatures down (running too lean raises head temps and can accelerate the
valve plastic deformation issue). Except for this fairly drastic throttle condition, I don't have any "pops", which
I think is fairly normal for these bikes. It helps me to be noticed by the multitudes of H-D riders around here,
who cannot hear my loud valves over their loud pipes !
