How carburetor works

Motorcycle carburetors works on similar principles: A passageway (Venturi) is connected to the intake side of the combustion chamber. A vertical tube (jets) runs vertically from the passageway to the float chamber. As the piston executes its intake stroke, a great vacuum is created in the combustion chamber and at the same time the intake valve opens that creates a suction pressure in the venturi and the jets. A simple explanation for this is like sucking the liquid inside a bottle with a straw while the remaining space inside the bottle is kept at atmospheric pressure.

The amount of fuel flow is controlled by the needle and the needle jet. The higher the needle is lifted, the greater the fuel flow. When the needle is at its lowest, it effectively shuts off the jet. At the highest position of the needle, gas flow is controlled by the smallest tube which is the main jet. The needle, needle jet, and main jet can be change to smaller or bigger size to adjust the fuel-air mixture ratio.

Airflow is controlled by a valve. Either a butterfly valve or an airslide. The airslide works hand in hand with the needle in such a way that when the needle is lifted allowing more gas flow, the slide opens allowing more air to flow. Airslide usually have a cutaway at the bottom to allow more or less air through at throttle openings.

The pilot jet with its own airway is built to provide a trickle of fuel for tick-over.  It is adjusted by a screw which opens or closes the size of the airway. This allows a smooth transition from a tick-over to faster running.

The fuel in the float bowl is kept at a constant level by the float. If the level is raised, it would cause too much gas to pass through the jets causing a rich mixture. If it is too low, it would cause a weak mixture. The fuel enters the bowl in an opening with a needle valve connected to the float.  When the float is lifted, it closes the entrance at a prescribed point and as the gas is being used it will go down with the level of the liquid gas, thereby opening the entrance to allow fuel to come in.