The idle screw is on the left hand side of the carbs when sitting on the bike (on mc18's anyway). I think you have it confused with the carb balance screw. Which can affect idle as well. There have been a few threads on this topic with one of the solutions being the carbs out of balance.

Here is a picture of where the idle screw goes. I have taken it out to use on a different set of carbs, but you can see where it presses on the throttle butterfly, the more you screw it in the more it forces the butterfly's open and vice versa.