All About the Carburetor

What does a carburetor do?

The carburetor has several functions: 1) it combines gasoline and air creating a highly combustible mixture, 2) it regulates the ratio of air and fuel, and 3) it controls the engine's speed.

How a carburetor mixes fuel and air

When the piston moves down the cylinder on the intake stroke it draws air from the cylinder and intake manifold. A vacuum is created that draws air from the carburetor. The airflow through the carburetor causes fuel to be drawn from the carburetor through the intake manifold past the intake valves and into the cylinder. The amount of fuel mixed into the air to obtain the required air to fuel ratio is controlled by the venturi or choke. When air flows through the venturi its speed increases and the pressure drops. This causes the fuel to be sucked into the air stream from a hole or jet. When the engine is at idle or at rapid acceleration there is not enough air passing through the venturi to draw fuel. To overcome these problems other systems are used.

Delivering gasoline to the carburetor

Gasoline is delivered to the carburetor by the fuel pump and is stored in the fuel bowl. To keep this level of fuel stored in the bowl constant under all conditions a float system is used. A float operated needle valve and seat at the fuel inlet is used to control the fuel level in the bowl. If the fuel level drops below a certain level the float lowers and opens the valve letting more fuel in. When the float rises it pushes the inlet valve against the seat and shuts off the flow of fuel into the bowl.


Controlling the speed of the engine

The throttle controls the speed of the engine by controlling the amount of air fuel allowed in the engine. The throttle is a butterfly valve located after the venturi and is opened by pressing on the gas pedal. The farther the valve is opened the more air/fuel mixture is let into the engine and the faster the engine runs. At low engine speeds when the throttle is only open a little there is not enough air flow to pull in fuel.


Two ports are used to solve this problem. One port located in the low pressure area and the idle port located below. At low engine speeds both ports draw fuel to keep the engine running. As engine speed increases fuel from the 2 ports decreases until it stops completely.

Handling low speeds

When the engine is idle there is very little air flowing through the venturi because the throttle valve is closed. The idle port allows the engine to operate under this condition. Fuel is forced through the idle port because of a pressure differential between air in the fuel bowl and vacuum below the throttle valve. Idle fuel mixture is controlled by an adjustable needle valve.

Handling high speeds

At higher engine speeds more fuel is drawn from the main nozzle. Fuel comes from the fuel bowl through the fuel nozzle and into the throat of the carburetor where it mixes with air.

Types of carburetors

There are 3 basic types of carburetors in use today. They are the one barrel, two barrel, and four barrel. Typically, the type of engine and its use will dictate which carburetor is used. In high performance engines multiple carburetors may be used to deliver the amount of fuel required. No matter what type of carburetor your engine uses, National Carburetors is your source for high quality carburetors.

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