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Simple carburetor consists of a float chamber, nozzle with metering orifice, venturi and
throttle valve.
The float and needle valve system maintains a constant height of petrol in the float
If the amount of fuel in float chamber drops below the designed level, the float lowers,
thereby opening the needle of fuel supply valve.
When designed level has been reached the float closes and needle valve stops the
additional flow of fuel from supply system.
The float chamber is vented to the atmosphere.
During suction stroke, air is drawn through the venturi.
Venturi has minimum cross-section at throat. Venturi tube is also known as ‘chock tube’
and is so shaped that it gives minimum resistance to airflow.
The air passing through the venturi increases in velocity and pressure decreases.
From the float chamber, fuel is led to discharge jet, the tip of which is located in the
throat of the venturi.
Because of pressure difference in throat and float chamber, the fuel from float chamber is
discharged at throat. This pressure difference is called as ‘carburettor depression’.
The rate of flow is controlled by the size of the smallest section in the fuel passage. This
is provided by discharge jet and size of jet is designed to give required engine performance.

The pressure difference at throat for full open throttle is about 4 to 5 cm of Hg below
atmosphere. To avoid wastage of fuel, the level of the liquid in the float chamber is kept at
short distance below the tip of discharge jet.

simple carburettor

The quantity of fuel discharged is regulated by butterfly type throttle valve.
As the throttle is closed, less air flows through the venturi and less air-fuel mixture is delivered to the cylinder and less power is developed.
As throttle is opened, more air flows through the venturi and the power developed increases.


A simple carburetor gives the correct mixture at only one engine speed and load.
Therefore it is suitable only for engine running at constant speed and load.
At very low speed, the mixture supplied is so weak that it will not ignite properly.

For this, arrangement must be made so that at starting it should give richer mixture.
In simple carburetor the mixture is weakened when the throttle is suddenly opened
because of inertia effect of the fuel, when mixture demand is of rich mixture.
The working of simple carburetor is affected by changes in atmospheric temperature and


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