There are usually two screws in a carb like this. One will be on the top of the carb and is the barrel stop. This will have a spring on it as you describe and it keeps the carb barrel from coming out of the carb as well as limit the movement of the barrel. The other screw will come out of the side of the carb on the same side as the throttle arm. There should be a hole in the carb at the end of the screw. You tune this in the same way as you do a low end needle valve, however try to make smaller movements than 1/16". I know that sounds difficult to do, but on an air bleed carb a little bit of adjustment goes a LONG way. I hate air bleed carbs. I've owned one and will never own another one ever again. It took me 5 days worth of working on it to get the idle set correctly with the air bleed. The only thing I can suggest is keep working at it, small movements of the adjustment, and be patient!!!

Ken