The carburetor's main function is to control the amount of air and fuel that goes into the engine in the right proportions at all engine rpm levels.

The carburetor utilizes air flow through the barrels to draw fuel out from the bowl(s). All carburetors work on two basic principles. One principle is the Bernoulli effect and the other is the venturi effect. Vacuum increases along with velocity in regard to The Bernoulli effect. If a carburetor is developed based on this principle it will have a fuel nozzle placed in the high velocity/low pressure (vacuum) region which allows the drawing of fuel into the air stream and atomizes it into a tiny mist of fuel droplets.

A carburetor that is developed and operates on the venturi effect will have a venturi (narrow passageway) in a tube that is responsible for increasing flow velocity. The carburetor will have a venturi area in the middle of the throat that increases velocity of the air stream. A booster venturi is placed in the main venturi area to further create air velocity plus create a region of very low pressure (vacuum). The fuel nozzle will be located in the booster venturi where there is the most vacuum. This allows fuel to be drawn into the air stream and atomized into tiny droplets. The position of the throttle plates/valves will determine how much air and fuel can enter the engine.

The needle and seat assembly acts as a shut off valve to maintain proper fuel levels in the carburetor float bowl at all times. As fuel is consumed the float will drop and allow the needle to drop off it's seat allowing more fuel to enter the bowl. As fuel enters the bowl it will raise the float level which will press against the needle assembly forcing it against the seat and shutting off fuel flow.

Each barrel of the carburetor will contain one main jet which acts as a restrictor that controls the amount of fuel that the carburetor can deliver to the engine. These main jets can be replaced with either larger or smaller jets depending on the fuel demands of the engine. The idle screws control the amount of fuel that enters the engine at idle. On a Holley carburetor, turning the mixture screws in leans out the air/fuel and turning them out richens the air/fuel mixture. The idle mixture screws should only be adjusted when the throttle blades/valves are nearly closed at engine idle. We'll cover carburetor tuning in another article.