How a Carburetor Turns Moving Air Into a Fuel Supply
A carburetor has to do two jobs at once: meter gasoline and break it into droplets small enough to travel with the air into an engine. Its key passage is a venturi, a section that narrows and then widens. As air is squeezed through the narrow part, it speeds up and its pressure drops.
Gasoline waits in a small reservoir called a float bowl. A float helps keep the fuel there at a fairly steady level. Because the bowl is at higher pressure than the fast-moving air in the venturi, that pressure difference pushes fuel through a small opening, or jet, into the airstream. The moving air breaks the fuel into droplets; the resulting mixture continues toward the engine’s cylinders.
Why the Throttle Changes the Mixture Flow
A throttle plate acts like an adjustable door in the air passage. Open it farther, and more air can flow through the carburetor, generally increasing the pressure difference that draws fuel through the jet. Close it, and airflow falls, reducing the supply. The jet’s size and the shape of the passage help determine how much fuel is delivered relative to the air.
This arrangement is mechanical rather than a direct measurement-and-control system: airflow itself helps draw fuel. That makes the carburetor compact and self-contained, but changing engine speed quickly can make the mixture harder to keep just right. The venturi’s pressure drop is doing the work—and also sets a limit on how freely air can pass through the passage.
