Feedback Control Solenoid: Description and Operation

Fig. 18 Feedback carburetor actuator:

This actuator, Fig. 18, is used to control the carburetor air/fuel ratio. The ECA uses inputs from sensors such as the exhaust gas oxygen, barometric pressure, etc., to calculate the correct signals to maintain a desired air/fuel ratio.
The FBCA is a stepper motor mounted on the right side of the 7200 carburetor. The FBCA has four separate armature windings, and each one is sequentially energized by the ECA in order to obtain the necessary control of the carburetor metering rods. The motor varies the position of the metering valve in the carburetor to regulate the amount of control vacuum exposed to the fuel bowl. The vacuum lowers the amount of pressure in the fuel bowl causing a leaner air fuel mixture.
During engine cranking and immediately after the engine starts, the ECA will set the FBCA at a calibration dependent upon initial position. The FBCA position is dependent upon ECA calculations of various engine sensor inputs.