Theory of the Venturi
The principle of pressure difference is the basis of carburetor function. A venturi is incorporated in all carburetors to establish this pressure difference. A venturi is a streamlined restriction in any passage. Such restriction causes an increase in velocity and a lowering of pressure of the air passing through it.
When the air strikes the leading edge of the venturi, it is deflected toward the center of the passage. Since all the air must pass through this smaller area, it attains a higher velocity than if it passed through a straight-walled cylinder. Due to the action of the air flow, a low pressure area is created in the venturi.
Air flows through the throat of a carburetor because of the lowered pressure created in the space above the engine piston as it travels downward in the cylinder. The action is exactly the same as shown in Figures 2, 3 and 4 (previous page). As the engine piston moves downward, the area above it is increased. This becomes an area of lowered pressure and the air from the atmosphere flows in to equalize the pressure. Fuel is added to this stream of air as it passes through the carburetor throat. The addition of the fuel is effected by the principle of pressure difference.