When I think back to the 1970s, I think of the controversial inventions Detroit came up with in its quest for fuel economy along with the challenge of meeting tougher federal emission standards. There were GM’s computer-controlled “feedback” carburetors and Chrysler’s Lean Burn “hot bake” processors inside the air cleaners.
There were also Ford’s 2700 and 7200 “Variable Venturi” 2V carburetors.
Despite the best of intentions at the engineering level, these engine “management” systems (both fuel and ignition) didn’t function very well and were cursed with technicians and repair shops that didn’t really know how to service the VV. The main problem was technical knowledge and how to tune this innovative carburetor.
They were not a bad design—just misunderstood.
Ford’s Variable Venturi (VV) 2700/7200 Series carburetor was a revolutionary design yet challenging to understand. When it was introduced in 1977, the VV atomizer was befuddling to even Ford technicians who were factory trained in its function and service.
…Now, we’re going to try to explain how the darned thing works.
How the Ford Variable Venturi Carburetor Works
On the surface, the VV carburetor looks like any other two-barrel carburetor—that is until you examine the throttle bores and discover you can’t see them. The venturi valves (which are connected to fuel metering rods) on top of the air horn block your view of the throttle bores when they’re at rest. Open the venturi valves and you can see the throttle bores and plates.
How the VV works is challenging to explain.
The VV has all the features of a conventional carburetor: a fuel inlet, bowl, and float; a main metering system for off idle function, an idle circuit, throttle plates and bores, and the variable venturi air valves. The venturi valves limit airflow, yet increase velocity depending upon how far they open.
Ford says, “As air flows through a restriction, its speed increases and pressure drops.” This is known as the venturi principle. Ford goes on to say, “In a conventional carburetor, this principle is applied through the usage of fixed venturis mounted in the venturi of the carburetor. In the VV carburetor, the venturi action occurs between the leading edge of the venturi valve and the carburetor casting.”
The venturi valves are actuated by the venturi valve diaphragm at the back of the VV carburetor. The venturi valve diaphragm is affected by intake manifold vacuum, which controls venturi valve function. This enables the venturi valves to match both air and fuel flow to meet any engine demand. The venturi valve looks like a secondary actuator on the Autolite 4100 4V carburetor, only it does something different. It operates the venturi valves.
Now—how this diaphragm operates requires further explanation. Where the diaphragm positions the venturi valves depends on engine speed and load according to Ford. This happens via an intake manifold vacuum signal, which is applied to the diaphragm from the carburetor throttle bore. This happens based on controlled vacuum in the throttle bores. The calibration for controlling this is based upon spring pressures versus the vacuum signal.
Like any carburetor, the VV has an accelerator pump to provide a fuel shot as the throttles open to prevent flat spotting and stalling. In linear fashion, the pump shot creates a rich shot to keep the fires burning as air flow increases. As the throttles close, the accelerator pump bore refills with fuel.
The VV carburetor does not have a choke like a conventional carburetor. Instead, it sports a Cranking Enrichment System. As its name implies, this enrichment system offers a rich mixture for cold starting via a thermostatic coil and fast idle cam to prevent stalling. As the engine warms, the coil brings back the fast idle cam for a normal idle and a leaner mixture. There’s also a choke pulldown diaphragm, which is actuated by intake manifold vacuum.
Changes, Alternatives & Replacements to the Ford VV Carb
Because the VV carburetor tended to be a work in progress, there are a number of changes to be aware of if you happen to be looking for one. Base your selection on the Ford part number and related information on the casting.
An alternative to the VV carburetor is conventional Autolite/Motorcraft 2100/2150 carburetor. Check your local smog laws to see if this swap is emissions-legal in your area.
If you choose to stick with a VV carburetor, the good news is you don’t have to rebuild it yourself. You can opt for a UREMCO remanufactured Ford Motorcraft VV carburetor found at places like Summit Racing, which has been professionally remanufactured under strict engineering standards. Rolling the dice on a remanufactured VV carburetor from your local auto parts store can be risky, and what kind of warranty are they offering you? United Remanufacturing Company (UREMCO) has over 60 years of carburetor remanufacturing experience. They have a terrific team of qualified technicians who have years of collective experience in the field.
All UREMCO Ford VV carburetors get an extensive reman process from disassembly, extensive cleaning, and testing to final assembly and inspection. They don’t need on-engine tuning when installed. Just install, fire the engine, and go. Do a leak check before hitting the road.