On all computer controlled (feedback) vehicles, the fuel flow through the carburetor idle main metering circuits is controlled by a mixture control (M/C) solenoid located in the carburetor. The M/C solenoid changes the air/fuel mixture to the engine by controlling the fuel flow through the carburetor. The ECM controls the solenoid by providing a ground. When the solenoid is energized, the fuel flow through the carburetor is reduced, providing a leaner mixture. When the ECM removes the ground, the solenoid is de-energized, increasing the fuel flow and providing a richer mixture. The M/C solenoid is energized and de-energized at a rate of 10 times per second.
The mixture control solenoid is mounted under the carburetor air horn assembly. Adjustment or replacement requires a partial rebuild of the carburetor unit. In the event of air horn removal, most of the carburetor internal and external adjustments must be checked and adjusted.