[nbpro]

Most Holley Street Performance, O.E. Muscle Car, Competition and Pro-Series HP carburetors utilize idle mixture screws, located on the sides of the primary metering block. These control the volume of the pre-mixed air/fuel coming through the idle well.
Turning the screws clockwise (IN) will “lean” the idle system.
Conversely, turning the screws counterclockwise (OUT) will “richen” the idle system.
The initial adjustment is made by turning the mixture screws in a clockwise direction until they lightly bottom. Back them both off 1-1/2 turns.
Connect a vacuum gauge to a carburetor vacuum port that will have access to full manifold vacuum at idle. Start the engine and allow it to warm up. Once the engine has warmed up and the idle stabilized, the choke should be disengaged.
When setting the vacuum it should be in gear.
Adjust the idle mixture screws to obtain the highest vacuum reading.
Each screw should be turned an equal amount so that the system is balanced.