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How a Carburetor Works

A carburetor is an essential part of older model internal combustion engine vehicles. The main function of a carburetor is to mix the fuel and air and feed it into the vehicle's engine where it is ignited and used to thrust the pistons downward inside the engine block. This force against the pistons is what creates power and causes a car to accelerate and maintain motion. The basic physics behind the function of a carburetor is called the Bernoulli Principle and the venturi effect. The Bernoulli Principle states that speed of the air is inversely proportional to the pressure. It is the throttle plate or butterfly of the carburetor that manages the amount of air flow that is delivered to the engine.

Carburetor - (appearance may vary)

The velocity of the flow and the subsequent pressure, gauges the quantity of fuel that is fed into the air stream. The venturi effect is an example of the Bernoulli principle. Fluid velocity must increase through the constriction in a tube. The kinetic energy is created by the drop in pressure. Carburetors use tubes called venturis to achieve this effect.

Below the venturi there is a valve called a throttle plate which can be open and closed by the throttle pedal. This controls the engine speed by restricting the air flow to the engine and subsequently the amount of air & fuel mixture that is delivered.

Mathematical Equation - (ρ/2)(v₂² - v₁²)

When the gas pedal is depressed, fuel is drawn into the air stream. These tiny holes are on the smallest section of venturi and called jets. When the throttle is opened the vacuum in the intake manifold is decreased. The velocity of air increases, the low pressure raises the air speed to draw additional fuel into the air stream through the nozzle located at the center area of venturi.

Other parts of the carburetor are an accelerator pump and choke assemblies. The accelerator pump forces a small quantity of gasoline through the jet when the throttle is first opened off of idle mode. This helps to eliminate the natural lean stumble created when an engine isn't moving enough air to draw the proper amount of fuel in during initial acceleration. An accelerator pump also primes the engine before cold starts.

On cold start conditions more fuel in needed to help the engine run properly. A choke is used to create this additional fuel. The air flow is controlled by a choke at the carburetor's entrance, before it passes to the venturi. Extra vacuum develops in the barrel of the carburetor, pulling additional gasoline via the main jet system. The choke also will hold the throttle plate open slightly to increase the idle speed to prevent stalling

Common Problems

The main problem with a carburetor is dirty fuel. This can cause a too rich or lean condition, meaning there is too much fuel or air to the mixture. The mixture is normally adjusted by one or more needle valves on the carburetor, but dirt can render these adjustments inoperative. To check your car's mixture you will need to use a gas analyzer to test your exhaust gases. This will measure the carbon monoxide, hydrocarbon and oxygen content of your exhaust. During a normal smog inspection the fuel/air mixture is tested and a report is generated. When a tune up is performed the spark plug end insulator can tell you a lean or rich mixture. Brownish grey is the desired color and confirms proper mixture. Black and sooty means the mixture is too rich while white to light grey confirms a lean mixture.

Basic maintenance

Changing the air filter and fuel filter will keep dirt/debris from entering the internal workings of the carburetor. This should be performed each time you change your spark plugs. Spraying a carburetor cleaner into your barrels and adding cleaning additive to your gasoline will remove deposits in your jets and in the carburetor itself.