The idle mixture screw controls the air/fuel mixture at idle. It is located under the carb throat (You do NOT remove the float bowl to access it) - closest to the engine.
That could be part of your problem as well. That little oracfice can get clogged pretty easily because it is small. Remove the screw (Keep an eye out for any washers and/or springs from there) and spray carb cleaner through there (Carb removed of course).

It works with the pilot jet at idle. To set it, Turn it all of the way in (Gently) and then back out 2.5 turns - that is your starting point. Turn it in until the engine starts to sputter. Then out until it starts to sputter - you want the most smooth and high idle - which will be directly in between those 2 sputtering points. You might need to adjust the idle speed back down in order to further adjust the idle mixture screw. Snap the throttle a few times. The engine should respond immediately without hesitation when it is set properly. Make sure to run the quad around for a few minutes, and then come back and check it again.