Carburetor Theory

The carburetor has one main function, to regulate engine speed. It accomplishes this by metering the amount of air and fuel as required, to sustain combustion per the input of the throttle servo. Thus for a low-speed idle you would have a small amount of air and fuel entering the engine. This would in effect lower the chemical energy entering the combustion chamber and thus lessen engine power and subsequently lower the RPM. As we open the throttle the carb will allow more air and fuel into the combustion chamber, thus increasing engine power and RPM's (revolutions per minute). Now that we know what the carb. has to do lets explore the underlining fluid mechanic properties that allow the carb to function effectively at different throttle settings.

Carburetor Theory

The Venturi-Effect

What allows the carb to pull fuel from the fuel tank is the venturi-effect. This states that in a converging funnel the entering fluid velocity increases as it passes through a reduction in the funnels throat diameter. This increase in fluid velocity decreases the localized pressure at the venturi throat to below atmospheric pressure. This low pressure region is precisely where fuel enters the carburetor throat. This is what allows the engine to "suck" fuel from the gas tank. The truth is that the venturi-effect is all that is needed for the engine to get fuel. Pressurizing the fuel tank is really only done to decrease the effects of fuel level on the mixture setting of the carburetor.

Fuel Metering Devices

The venturi-effect draws fuel from the tank but does little to regulate it's flow. It's true that as the engine accelerates the amount of air that moves through the engine increases. The increase in air velocity also increases fuel flow into the induction port, this helps the engine self regulate the fuel up to a certain point.

This is not the only means for the carburetor to meter air and fuel. Engines need a metering device to help regulate the amount of fuel that enters the carburetor. This is accomplished with an adjustable orifice, typically we call them needles or jets. Most engines have a second adjustable needle that helps regulate fuel at low throttle settings. By adjusting these two needles we can control the transition from low to high speed operation of the engine.

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