Super Custom ??? Anyway the screw on top of carb usually has a spring on it to prevent turning from vibration. If this is one I think it is it does 2 things. 1- keeps throttle barrel from falling out. 2- limits amount throttle can close. That's the one you should be able to turn out slight amount to allow throttle to close more. turn it enough so barrel can totally close off carb throat so you can stop engine at end of flight by using trim to close fully. However limit amount of throw using throttle end point adjustment to provide proper idle speed. Do that with throttle trim in neutral then to stop engine trim all the way down.