Since the design of the 2700VV (variable venturi) carburetor differs considerably from the other carburetors in the Ford lineup, an explanation in the theory and operation is presented here.
In exterior appearance, the variable venturi carburetor is similar to conventional carburetors and, like a conventional carburetor, it uses a normal float and fuel bowl system. However, the similarity ends there. In place of the normal choke plate and fixed area venturis, the 2700VV carburetor has a pair of small oblong castings in the top of the upper carburetor body where you would normally expect to see the choke plate. These castings slide back and forth across the top of the carburetor in response to fuel-air demands. Their movement is controlled by a spring-loaded diaphragm valve regulated by a vacuum signal taken below the venturis in the throttle bores. As the throttle is opened, the strength of the vacuum signal increases, opening the venturis and allowing more air to enter the carburetor.
Fuel is admitted into the venturi area by means of tapered metering rods that fit into the main jets. These rods are attached to the venturis, and, as the venturis open or close in response to air demand, the fuel needed to maintain the proper mixture increases or decreases as the metering rods slide in the jets. In comparison to a conventional carburetor with fixed venturis and a variable air supply, this system provides much more precise control of the fuel-air supply during all modes of operation. Because of the variable venturi principle, there are fewer fuel metering systems and fuel passages. The only auxiliary fuel metering systems required are an idle trim, accelerator pump (similar to a conventional carburetor), starting enrichment, and cold running enrichment.
The Motorcraft model 7200 Variable Venturi (VV) carburetor shares most of its design features with the model 2700VV. The major difference between the two is that the 7200VV is designed to work with Ford's Electronic Engine Control (EEC) feedback system. The feedback system precisely controls the air/fuel ration by varying signals to the feedback control monitor located on the carburetor, which opens or closes the metering valve in response. This expands or reduces the amount of control vacuum above the fuel bowl, leaning or richening the mixture accordingly.