Ford Variable Diffuser Carburetor

Ford close-throttle carburetor cutaway

Cross-section of a Ford open-throttle carburetor

Carburetor Ford W (Motorcraft Venturi Variable), also called variable diffuser carburetor, single-chamber, with a float chamber and a variable-section fuel nozzle controlled by vacuum.

The vacuum formed in the intake manifold due to the operation of the engine controls the opening of the diffuser flap, as well as a change in the cross section of the fuel jet, closed by a calibrated conical needle. This carburetor has an enrichment device during cold start, automatically activated and controlled by a bimetallic spring that responds to temperature changes. The accelerator pump, which enriches the mixture when the throttle is quickly opened, is also controlled by vacuum.


The main fuel metering system consists of the main and additional jets, as well as a calibrated conical needle that regulates the fuel supply to the carburetor diffuser. Fuel comes from the carburetor jets under the influence of a vacuum formed in the diffuser through which air passes (between the main jet and the movable shutter, see fig. Ford closed throttle carburetor in section and fig. Cross-section of a Ford open-throttle carburetor).

With a low load on the engine, the needle connected to the movable shutter almost completely closes the main fuel jet. As the load on the engine increases, the vacuum increases and the movable damper gradually opens the diffuser section. The needle moves along with it, increasing the cross section of the fuel flow, and an additional fuel jet opens. The needle is made with great precision and is an important element of the exact dosage of fuel depending on the load on the engine. Should not be regulated (change setting) needles.

During idling, when a small amount of air enters the engine, the movable damper is in the extreme closed position and is held by the vacuum regulator diaphragm return spring. As the diaphragm opens, the intensity of the flowing air flow increases and the vacuum inside the diffuser increases. This vacuum, acting on the regulator, causes the regulator flap to move, and with it the movable flap and needle. The position of the diaphragm at each moment of time is determined by the balance of the forces acting on it: the vacuum and the return spring.