If that is a curb idle screw such as most AFB's use, then turn the idle air screw all the way closed, and set the idle using only the idle mixture screws and the curb idle screw.
If you have a big cam then the idle air screw becomes an adjustable idle air bleed.
If you have a fairly normal cam, then leave the idle air screw completely closed, and use the curb idle screw.
Jon
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"Good carburetion is fuelish hot air". 
"The most expensive carburetor is the wrong one given to you by your neighbor".
If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!
Owner of The Carburetor Shop, LLC (of Missouri).
Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
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