Mixture Control Solenoid: Description and Operation

Fuel flow through the carburetor idle/main circuit is controlled by MC solenoid controlled metering rods. The MC solenoid controls air/fuel mixtures by allowing more or less fuel to flow through the carburetor. The ECM controls MC solenoid operation by completing or breaking the solenoid ground circuit.
When the MC solenoid is energized (down position), fuel flow is reduced, providing leaner mixtures. When the solenoid circuit is open (up position), more fuel is allowed to flow, providing richer mixtures. The ECM cycles the MC solenoid ``on and off'' at a fixed rate of 10 times per second, but varies the amount of time that the solenoid is energized per cycle (dwell period) to control air/fuel mixture.