Be careful with this screw. They are often frozen and will strip, or break easily. If the screw doesn't move easily to start with, try heating around the outside of the screw. This will usually loosen it up.
Tighten the screw until it touches the secondary throttle lever, then tighten one more turn.
If you are having problems getting the primary idle mixture and/or idle speed just right, try turning the screw in just slightly. This will allow the primary plates to close enough to cover the idle transfer slots.