(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
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.)