There is always unburnt fuel in the exhaust. There is also heat.
Combustion (banging) does not usually happen in the exhaust of a well-tuned engine because there is not enough oxygen present.
An air leak into the exhaust near the valve allows enough oxygen in, to support combustion. The effect is loud banging on over-run.
A weak pilot mixture can have a similar, but usually not so loud, effect. The oxygen than was drawn in on the inlet stroke has not all been used.
Needle position shouldn’t have much, if any, effect with the throttle closed.