Just to be sure...
You cleaned the little holes in the jets?
Also, as spiider suggested, did you clean the screen on the petcock?
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Popping or backfiring at idle is probably a symptom of it being lean. If your airscrew is on the airbox side close it a bit.
What you can do is warm up the bike, set it to a fast idle, and adjust the airscrew to the max rpm (by ear in your case).
Once it is adjusted for max rpm...that's it for the airscrew.
Also, it won't be the main jet...the main is responsible for 1/2-full throttle more than anything else.
The pilot/slow jet is responsible for the idle to 1/4 throttle....then the needle from 1/4 to 1/2 or so...then the main take it to redline.
(They all overlap a tad.)
^ +1
See the video below, this guy explains carburetor operation and the adjustment process very well.
How to adjust the jets on motorcycle carburetors
http://www.youtube.com/watch?v=y836ZPjvoMI