The function of a carburetor is to vaporize the petrol (gasoline) by means of engine suction and to supply the required air and fuel (petrol) mixture to the engine cylinder. During the suction stroke, air flows from atmosphere into the cylinder. As the air passes through the venturi, velocity of air increases and its pressure falls below the atmosphere. The pressure at the nozzle tip is also below the atmospheric pressure. The pressure on the fuel surface of the fuel tank is atmospheric. Due to which a pressure difference is created, which causes the flow of fuel through the fuel jet into the air stream. As the fuel and air pass ahead of the venturi, the fuel gets vaporized and required uniform mixture is supplied to the engine.
The quantity of fuel supplied to the engine depends upon the opening of throttle valve which is governed by the governor.
The main parts of a simple carburetor are:
Float chamber: The level of fuel in the float chamber is maintained slightly below the tip of the nozzle. If the level of petrol is above then the petrol will run from the nozzle and drip from the carburetor. If the petrol level is kept low than the tip of the nozzle then part of pressure head is lost in lifting the petrol up to the tip of nozzle. Generally it is kept at 5mm from the level of petrol in the float chamber. The level of the fuel is kept constant with the help of float and needle valve. The needle valve closes the inlet supply from main tank if the level rises above the required level. If the level of fuel decreases then the needle valve opens the supply. Generally the fuel level is kept 5mm below the nozzle tip.
Venturi: When the mixture passes through the narrowest section its velocity increases and pressure falls below the atmospheric. As it passes through the divergent section, pressure increases again.
Throttle valve: It controls the quantity of air and fuel mixture supplied to the engine through intake manifold and also the head under which the fuel flows.
Choke: It provides an extra rich mixture during to the engine starting and in cold weather to warm up the engine. The choke valve is nearly closed during clod starting and warming. It creates a high vacuum near the fuel jet which causes flow of more fuel from the jet.