Model 6500 Feedback Carburetor

This unit is basically the same as a Model 5200 carburetor except it has an externally-variable auxiliary fuel metering system in place of the enrichment valve used on the 5200. The auxiliary system is operated by vacuum from the vacuum solenoid regulator under control of the ECU.

Control vacuum from the vacuum solenoid regulator is channeled to the cavity above the metering rod diaphragm. 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. This reduces the orifice area and permits less fuel to pass into main well tube passage of carburetor.

The metering valve is calibrated so that the maximum vacuum signal (5 in. Hg) supplied by the vacuum solenoid regulator raises the metering rod to its highest (leanest) position. See Figure.

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

During wide open throttle, 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 for good engine performance.

The 6500 carburetor has a switching bowl vent that allows venting fuel bowl vapors to the canister and a fresh air pickup for the vacuum solenoid regulator. This carburetor also has an internal choke pulldown diaphragm vacuum supply instead of the external vacuum connection on the 5200.