The air/fuel mixture screws meter the fuel that flows under each primary throttle plate when the throttle is closed at engine idle. Both of the air/fuel mixture screws control fuel flow.
Turning the mixture screws "in" will decrease the fuel flow and "lean" out the air/fuel mixture, and turning the mixture screws "out" will increase the fuel flow and "en-rich" the air/fuel mixture.
To adjust the air/fuel mixture warm up the engine to normal operating temperature and with the choke off at normal idle speed. Adjust the air/fuel mixture screws one at a time, and turn each air/fuel mixture screw "in" until the engine just begins to mis-fire, then back the air/fuel mixture screw "out" 1-1/2 - 2 turns a half turn at a time until the idle is smooth.
If you need a base setting to start with, then turn both air/fuel mixture screws "in" until they lightly seat, and then back them "out" 1-1/2 - 2 turns and then start and warm up the engine and then proceed to adjust the air/fuel mixture as described above.