The function of the Carburettor is to produce the fuel-and-air mixture needed for the operation of a petrol engine.

In the carburettor the fuel is distributed in the form of tiny droplets in the stream of air. As a result of heat absorption on the way to the cylinder these droplets are vaporised, so that the mixture thus becomes an inflammable gas.

The vapour-and-air mixture thus formed enters the combustion chamber of the cylinder. A modern carburettor comprises four different systems: the main carburettor, the idling (slow-running) system, the acceleration pump, and the choke.

Combustion of the mixture in the cylinder requires oxygen; this is present in the air drawn into the cylinder when the piston descends. The air is passed through a filter (air cleaner) and a pipe called the induction pipe or intake manifold, in which the petrol is carried along with the stream of air.

This effect is based on a law of physics known as Bernoulli's equation, which states that for a gas (or other fluid) flowing through a pipe the sum of the static pressure and the dynamic pressure is constant. This means that when the velocity, and therefore the dynamic pressure is increased, the static pressure decreases.

If the induction pipe is narrowed to a reduced diameter at one particular section, the velocity of the air at that section will be increased, while the (static) pressure will diminish to a negative value in relation to the surroundings. In other words, a suction is developed there, which causes the petrol to be sucked out of the choke tube and be atomised.

The tiny droplets are carried along into the cylinder by the air stream. The air intake pipe is preferably laid close along the exhaust pipe. The heat given off by the latter serves to preheat the intake air and helps to vaporize the petrol droplets. The main jet, which is essentially a constriction in the fuel supply pipe from the float chamber to the choke tube, limits and controls the quantity of fuel introduced into the air stream per unit of time: the jet has a small orifice which allows the fuel to flow from the float chamber only at a certain limited rate.