I also found this little piece.
Deceleration Pooping/Hanging Idle
Popping: I frequently hear someone say “I have popping in the exhaust under deceleration so my carb must be lean!” Deceleration popping tells us a couple things;
1: Inside the exhaust pipe is hot.
2: Idle circuit is allowing fuel into the engine.
3: Somewhere the fuel in the exhaust pipe (which is not burnt as we are not allowing oxygen into the engine) is coming into a source of oxygen! Thus finding some air so it can explode!
That’s it! It does not mean its lean! You can richen up the idle mixture until the exhaust is no longer hot and the popping may go away? But now you have a cooler combustion chamber and the AC pump is going to have more trouble getting the engine to respond to the twist, you may find yourself going around in circles?
If you want to understand the engines function better? The engine is a big air pump, as long as the engine is turning over it is trying to draw air in. That airflow is blocked by the throttle slide so under deceleration there is very high vacuum on the front side of the slide.
What is happening is a combo of two things:
1: High vacuum at the intake port is sucking fuel from the idle circuit, that tiny orifice forward of the slide so some fresh fuel is being run thru the engine, as the air supply is almost closed off this fuel goes thru the engine mostly unburned. (BTW, several years ago when cars got FI and the mileage improved! The reason the mileage improved is the FI shuts off the fuel when the vehicle is driving the engine).
2: Under deceleration high vacuum as the exhaust valve opens before BDC at the end of the power stroke it draws air from the exhaust back into the engine. The exhaust had momentum going out, suddenly there is suction making it reverse. At that moment the suction at the exhaust valve area is so high that the tinniest leak at the head pipe will draw in fresh, dirty air!
“Fresh” in that it has lots of Oxygen! This ignites the un-burnt fuel out in the exhaust port!
Dirty in that it brings with it dirt that is around the front of the cylinder head, this gets under the valves and causes some minor pitting of the valve. Not catastrophic but when I take a head apart I have a pretty good idea of how good your head pipe seal is! (it has been a growing problem since the Siamesed pipe design that KTM went to in 04).
Dave’s way;
1: Do not use popping as a gauge to lead you to modifying your jets,
2: Remove the pipe, seal its inside with high temp silicone, let it dry a few days before you start the engine.
