After your car is fully warmed up, set the idle speed first. Most carburetors have an idle set screw. This is a screw that contacts the throttle linkage. Some have an air bleed screw that lets in additional air. This screw is rather large and it’s located in the base or body of the carburetor. You want the idle speed at about 800 RPM in neutral for 8 cylinder engines, 850 RPM for 6 cylinder engines, and 900 to a 1,000 RPM for 4 cylinder engines.
Next you adjust the idle mixture screw/s. One barrel and progressive 2 barrel carburetors will have just one. Two and four barrel carburetors will have two. These are located in the base of the carburetor. The starting point for the adjustment for most carburetors is 3½ turns out from lightly seated (never torque a mixture screw tight, only lightly screw it in to find the seat). This screw regulates fuel, and screwing it out is more fuel (richer), and screwing it in is taking fuel away (leaner). For 2 and 4 barrels with the engine running and at idle, start with either screw and turn it in a ½ turn (clockwise). Then go to the other and turn it in ½ a turn. You want to keep them equal.
Keep going in back and forth between the 2 screws until the engine starts to shake just a little (because the mixture is getting too lean), then back out both screws ½ to ¾ of a turn until it smoothes out. For a one barrel carburetor, turn the screw in slowly until the engine just starts to falter, then back it out 1/2 to 3/4 of a turn.