Tuning question

Hey everyone.
My '77 750B is now running reasonably well after lots of time and love. It has a strange two-into-one that "seems" to flow with little restriction if the sound is any indication. It also has a oiled foam air filter in the stock box. And a pair of the cleanest carbs known to human kind.
When the throttle is opened good sounds come out. When the throttle is closed bad sounds come out. Popping and banging.
Typically I would raise the needle but it's already second to the lowest setting. The pilot screws are two full turns out. The plugs look great when chopped at 4K RPM.
Is the next step to richen the main jets?

Un burnt fuel igniting in the exhaust on over run is perfectly normal. With a complex standard exhaust you tend not to hear it. With a straight through exhaust you can especially if you have an air leak in the exhaust system.