I have a 2000 scrambler and it would idle kinda ruff. Then i turned in the screw with the spring on it on the carb now it idles fine. What screw is this? is is for fuel or air? the smaller set screw just aft of it what is it for?

 

The screw with the spring is the idle screw. Turning it simply changes the idle speed up or down. The other screw you mentioned is the air screw. Turning it in richens the mixture for the idling circuit, turning it out leans the mixture.