Fuel Delivery and Air Induction: Description and Operation

Carburetor:

WARNING: Do not smoke, carry lighted tobacco or an open flame of any type when working on or, near any fuel related component.

The Motorcraft 7200 VV (variable venturi) feedback carburetor is able to vary the area of the venturi according to engine speed and load, making it different from other Motorcraft carburetors which have a fixed area venturi. This is accomplished by dual venturi valves controlled by engine vacuum and throttle position. Depending on the engine demands, the position of the venturi valves change to determine the area for airflow to the two throats of the carburetor.

The venturi valves are connected to two tapered main metering rods which ride in the main meter jets. When the venturi valve position varies, the metering rods vary the fuel flow by changing the flow area of the main metering jets. In operation, air speed through the carburetor is fairly constant making for more even air/fuel mixtures throughout the engine operating range. In a fixed venturi carburetor, airflow speed varies according to throttle opening and engine speed. Thus, in order to work with the changing flow speed, supplementary systems such as an independent idle system and power enrichment system are needed. In the Motorcraft variable venturi carburetor supplementary systems are not necessary.

The feedback carburetor varies the air/fuel ratio in response to command from the MCU control module. The air bleed feedback system employed on the 7200 VV model uses a stepper motor (activated by a signal from the MCU module) to regulate bleed air admitted into the main fuel system. The stepper motor modulates the pintle movement in the metering orifice, thereby varying the air bleed into the main system. The greater the amount of air, the leaner the air/fuel mixture.