Which screw? On the main throttle control or on the carb itself? The one on the throttle control is the "governed speed adjustment", the one on the carb (black plastic screw) is the "true" idle adjustment.
If you're still stuck try adjusting the fuel/air mixture screw, if it's set a little lean you'll get a high idle. About a 1mm crack of the throttle being open and 1-1/2 turns out (from lightly seated) of the mixture screw is a good starting point.
Also make sure your mounting gasket and carb insulator "plate" are oriented correctly, if it's not you can have a vacuum leak (essentially).
As a quick test what happens when you close the choke while the motor is running?