[351mustanger]

First off, eddie is right that you should turn the screw in until lightly seated (while counting the number of turns it takes to get there). If your adjustments are unsatisfactory, you can always go back to the original setting.

1. The air/fuel screw lets an already mixed amount of fuel and air into the carburetor to be mixed with the fuel coming in through the pilot jet.

2. Since the air/fuel screw mainly works with the pilot jet (idle to 1/4 throttle), the main jet will have a very minimal effect on its setting.

In my experience, the air/fuel screw can tell you how close your pilot jet is. To set your air/fuel screw: warm up the engine to operating temperature, turn the air screw in or out until you reach the highest idle (or if there is somewhere to hook up a vacuum gauge, the highest vacuum). Once you reach the highest idle, that is your optimum setting. Then turn the screw back in until lightly seated while counting the number of turns. The screw should be somewhere between 1 and 2 turns out from lightly seated. If it is less than 1 turn, the pilot jet is too big. If it is more than 2 turns the pilot jet is too small. Once you reach the highest idle, or vacuum, adjust the idle back to standard with the idle adjustment screw.