A basic carbureted system consists of:
The carburetor turns liquid fuel into a fine spray and mixes it with air, and it has to supply the correct mixture of air and fuel to suit all operating conditions.
It also controls how much air-fuel mixture is delivered to the engine. This is done by the throttle valve near the bottom of the carburetor, which is connected to the accelerator or gas pedal.
The downdraft carburetor is the most common kind. The side-draft model is less common.
The down-draft carburetor is usually mounted on the intake manifold, and it has a float bowl for fuel. One end of a tube is immersed in the fuel. The other end is a fuel discharge nozzle, in the venturi. As the piston moves through its intake stroke, it makes a low pressure area and as a result, air from the atmosphere flows through the venturi. The venturi here is narrower than the rest of the barrel, and it is shaped to make the air speed up as it passes through.
A similar effect occurs around the wings of aircraft. The shape of the wing section speeds up the airflow over the top of the wing, and creates a low-pressure area there, lower than the atmospheric pressure below. The result is an upward force that provides lift for the aircraft.
The shape of the venturi is designed to apply the same principle, known as the 'Bernoulli effect'. It creates a low pressure area where the end of the nozzle protrudes into the airflow. Atmospheric pressure on fuel in the float bowl is now greater than the pressure on the end of the nozzle. This forces fuel to flow from the nozzle. It mixes with the passing air, breaking up into droplets, or atomizing.
A light vehicle under normal conditions needs an air-fuel ratio, by mass, of about 15 to 1. That means that the ratio is 15lbs or 15Kg of air for every pound or kilogram of fuel. Because air is much lighter than fuel, that same ratio of air to fuel appears much greater when looked at by volume. The volume of air to fuel needed is 11,000 gallons or liters of air to every one gallon or liter of fuel – a ratio of 11,000:1.
This ratio can vary to suit engine operating conditions. Too much fuel for the volume of air will waste fuel and cause pollution. Too little will cause loss of power and possible engine damage.