Motorcraft 6500 2 Barrel

Fig.6 - Feedback Carburetor:

Fig.7 - Feedback Carburetor Metering System:

The model 6500 feedback carburetor, Fig. 6, has an externally variable auxiliary fuel metering system in place of the enrichment system used on other carburetors. The auxiliary system is operated by vacuum from the vacuum regulator/solenoid under control of the ECU. A switching bowl vent is also used, which allows venting of fuel bowl vapors to the vapor canister and fresh air pickup for the vacuum/regulator solenoid.

A metering valve assembly, Fig. 7, operating piston and diaphragm assembly, is used in place of the fuel enrichment valve used on other carburetors. The feedback metering valve supplements fuel entering the main well through the conventional main metering jet and channel. The amount of fuel entering the main well through the metering valve depends on the position of the tapered metering rod in its orifice, which is controlled by the operating piston assembly.

Control vacuum from the vacuum regulator solenoid is routed to the cavity above the metering rod diaphragm, Fig. 7. With no vacuum present, the valve spring causes the valve to move to its lowest (richest) position, where maximum fuel can pass through the orifice. As vacuum is applied to the diaphragm, spring pressure is overcome and the metering rod rises, reducing the orifice area, allowing less fuel to pass. The metering valve is calibrated so that the maximum vacuum signal (5 inches Hg) supplied by the vacuum regulator/solenoid raises the rod to its highest (leanest) position.

In the open loop mode, a constant vacuum of 2 1/2 inches Hg is applied to the feedback valve diaphragm, holding the metering rod at its mid-position. In the close loop mode, the vacuum signal varies from 0 to 5 inches Hg, producing a richer or leaner mixture in response to signals from the ECU.

During wide open throttle acceleration, manifold vacuum drops, resulting in little or no output vacuum signal. The feedback metering valve moves to its lowest (richest) position, providing a richer air/fuel mixture in conjunction with the main metering system and thus, the correct mixture for good engine performance.

The feedback carburetor has an internal choke pulldown diaphragm vacuum supply instead of the external vacuum connection used on other carburetors.