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Years ago, I had a Carter AVS 4 bbl on my '69 roadrunner. This carb had 3 idle mixture screws! The principle is still the same. It's not a good idea to mess with them unless you have some idea of what you're doing. But a rule of thumb is hook up a vacuum gauge to the intake(a tee connection) and, starting with the screws turned very lightly seated in(clockwise), turn them out equal amounts in 1/8 of a turn increments until you get the highest vacuum reading. Alternately, you can turn them in with the engine running until you first detect the engine rpm's dropping then turn them out til you hear the engine slow down or miss slightly. The ideal position is in between the two settings. Make sure you don't have any vacuum leaks elsewhere at the carb base or intake. The choke has to be fully off or open also. The air bleeds can be sprayed with carb cleaner. Sometimes this will smooth out a rough idle. If the idle becomes too high while adjusting the mixture, turn the idle speed screw down, and start over.
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