The idle system supplies the air/fuel mixture to operate the engine at idle and low speeds. Fuel enters the main well through the main metering jet that is screwed into the metering block. Some of this fuel is then bled off to an idle well where it is mixed with air from the idle air bleed hole. The idle well leads directly to the idle discharge port and the idle transfer system where this air/fuel mixture is discharged.

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 will "lean" the idle system. Conversely, turning the screws counterclockwise 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. 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.

Idle System.
Model 2300 - Primary Side 4V.

Curb Idle System.
Primary Idle Transfer System.