With these SOHC4 engines, the cam timing has quite a lot of overlap (i.e., the valves are both open at once between the exhaust and intake stroke), which is a little unusual in the world of car engines or DOHC bike engines. Thus, there are some slightly different characteristics and symptoms to get used to (which sometimes causes confusion...).
First, Flybox: you are correct in that the decel is popping due to extra fuel leftovers. This is because the decreased backpressure is causing a 'non slowdown' of the inlet flow when you first shut off the throttle, and a surge of unburned fuel reaches the hot header pipes. This quickly vaporizes and the subsequent leaner hot pulses set it off, making the popping noise. If the sprakplugs are staying clean enough, there is no harm, but if they are starting to look darker, than first try turning the idle air screws in a little at a time, and retest (always with a warm engine). Don't turn them in large increments, like 1/4 turn at a time, or you'll add more confusion: their active range is only about 1.5 turns wide, i.e., 3/4 turns more open or closed from the factory setting. Beyond that range, they do strange things because of the way these carbs work. They are not like automotive carbs.
If, for example, the screws get too far in, the affected cylinder gets too lean to fire at all. But, since the next cylinder in the firing order can drag it along anyway, this pumps some of the unburned fuel into the header, and some gets left in the bore, so the NEXT time there will be enough to fire. This action then drags the previous cylinder through the same thing, and the exhaust "appears" to be rich again, when in fact it is very lean. Even a CO meter then shows a higher reading, because the exhaust mix has a bunch of unfired loads being pumped into it. This REALLY confuses folks...
If the screws go too rich (open), then the cylinder can't burn it all. In as few as 3 engine cycles, this makes the plug wet and the richness in the pipes goes way up, but there is not enough "fire" in the pipes to set it off an make it pop until the bike has coasted a long(er) way down a hill, or something similar. Then it starts popping.
So, if you have long hills around, you can use them to help diagnose things. Here in Colorado, that's REAL easy to find.
So, just try adjusting those screws a bit at a time, and you may well hit the right spot with the stock parts you already have. The setting will just not be stock.
