Welcome to the forum. Hope you enjoy it as much as we do. I'll try to help you here but in the future please post technical questions in the 'Trouble Shooting' section where it will get faster response.
The small screw you mentioned is only to set/adjust the engine idle speed. It screws in to raise the idle and out to lower it. If your engine is running too fast and the screw does not help the next place I would look is at the carburetor slide or barrel. Remove the air cleaner and look down the throat of the carb while twisting your throttle grip. You should see a dark colored cylinder lifting and lowering. Often the problem is a kinked or misadjusted throttle cable. If you had the carburetor apart its also possible that the slide/barrel was installed backwards. There are two slots in it. The longer slot goes to the right side of the carb as viewed from the top looking down. The shorter slot, the idle speed slot goes to the left. One other thing that will cause a high idle speed is an air leak in or around the carburetor and/or the intake manifold. Make sure you have a good seal where the carb attaches to the manifold and the manifold to the cylinder. Take a look at these things and get back to us with what you find.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"