Ford closed throttle carburetor in section
1 – throttle lever, 2 – movable diffuser flap, 4 – main fuel jet, 5 – additional fuel jet, 6 – float chamber, 7 – mixture adjustment screw, 8 – throttle stop screw for adjusting idle speed
Ford carburetor with open throttle in section
The Ford W (Motorcraft Venturi Variable) carburetor, also called a variable-section diffuser carburetor, is a single-chamber carburetor with a float chamber and a variable-section fuel atomizer controlled by vacuum.
The vacuum created in the intake manifold by the engine operates the diffuser flap and changes the cross-section of the fuel jet, which is closed by a calibrated conical needle. This carburetor has a device for enriching the mixture during cold engine starting, which is automatically activated and controlled by a bimetallic spring that reacts to temperature changes. The accelerator pump, which enriches the mixture when the throttle valve 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 is supplied 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 valve, see Fig. Ford carburetor with closed throttle valve in section and Fig. Ford carburetor with open throttle valve in section).
At low engine load, the needle connected to the movable flap almost completely closes the main fuel jet. As the engine load increases, the vacuum increases and the movable flap gradually opens the diffuser cross-section. The needle moves with it, increasing the fuel flow cross-section, and the additional fuel jet opens. The needle is manufactured with great precision and is an important element of precise fuel dosing depending on the engine load. The needle should not be adjusted (changed in setting).
During idle running, when a small amount of air enters the engine, the movable flap is in the extreme, closed position and is held by the return spring of the vacuum regulator diaphragm. As the diaphragm opens, the intensity of the 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 the needle. The position of the diaphragm at any given moment is determined by the balance of the forces acting on it: the vacuum and the return spring.