See Figure 1
In addition to the oxygen sensor, the EFC uses an Oxygen Feedback Solenoid. Its purpose is to regulate the air/fuel ratio of the feedback carburetor, along with a conventional fixed main metering jet, in response to the electrical signal generated by the Spark Control Computer.
With the feedback solenoid de-energized, the main metering orifice is fully uncovered and the richest condition exists within the carburetor.
With the feedback solenoid energized, the solenoid pushrod seals the main metering orifice. This position offers the leanest condition within the carburetor.