Main dosing system

Main dosing system

Float Needle Valve

Fuel enters the carburetor through a needle valve controlled by a float. When the fuel level in the float chamber is low, the float drops and opens the needle valve, as a result of which the float chamber is filled with fuel, the float rises and closes the needle valve, stopping the flow of fuel.

The fuel level in the carburetor is not adjustable, since small fluctuations in the fuel level do not affect the operation of the carburetor. The opening of the float chamber communicates with the body of the main jet and the suction channel of the carburetor, which eliminates the possibility of gasoline vapors entering the atmosphere.


The composition of the working mixture is regulated by a pneumatic valve, which opens and closes depending on the engine operating mode. This valve is actuated by a diaphragm, which in turn opens and closes depending on the vacuum in the diffuser between the pneumatic valve and the throttle valve.

During idling, the engine consumes little air and therefore the valve closes, due to which the air velocity near the outlet of the main jet increases. However, when the throttle valve is opened, the vacuum in the diffuser increases and the diaphragm held by the spring opens the pneumatic valve.

With further opening of the throttle valve, the valve continues to open and the force of the control vacuum and the spring tension are balanced over the entire speed range. Fuel is drawn from the float chamber through the transfer tube and then through two jets and a conical needle valve is sent to the engine.

At low speeds, the needle enters the main jet and limits the flow of fuel. During acceleration and at high speeds, the needle is pulled out of the main jet by the pneumatic valve to which it is attached. Because the needle is tapered, the amount of needle travel controls the amount of fuel that passes through the main jet.