there are 2 idle adjustments .. one is idle RPM.. the other is idle fuel/air mixture.
The idle speed screw mechanically pushes on the throttle linkage a bit.. It does the same thing as slightly twisting the throttle on the handle bars.
The other little screw is the air/fuel mix. At idle, a engine requires a rich mixture and this screw regulates how much air is mixed with the fuel at idle. The combination of this air-adjustment screw and the hole whose opening it regulates is your idle jet.
These two adjustments work together. The object is to lower the idle speed as much as possible. i'll paste something from another thread..
Regarding crappy idling in general:
your idle air/fuel mixture is wrong for some reason. It could be because of a vacuum leak or it may need nothing more than a simple adjustment of the idle speed screw and/or idle air/fuel ratio screw.
Vacuum leaks can happen anywhere inbetween the carb and the engine block. I recommend using a coating of Permatex 2 (non-hardening gasket sealant) on all the joints.. the carb to the intake manifold, the manifold to the reed valve block, and the valve block to the head. Use a small brush and put a thin coat on both sides of all mating surfaces.
As far as adjusting idle screws, drive a mile or so to warm up the bike to normal operating temperature.. then turn the speed screw in until the motor spins fast enough to idle. (this speed screw simply mechanically opens the throttle linkage.. the other little screw down there is the mixture screw)
Now, slowly turn the idle air/fuel mixture screw in and out until you notice a point where the engine speed increases. Next, back off the speed screw and slow the motor down until it threatens to die.. Then turn the mixture screw in and out till the motor speeds up..
At this point you may be able to slow the motor even more with the speed screw. If so, again adjust the mixture screw until the engine speeds up.
Test ride the bike and see if it the engine rpm drops to a smooth, slow idle. If not, repeat the adjustment proceedure.
i hope that helps.