What is the back-suction mixture control system in carburetor?

The Back-Suction Mixture Control System consists of a venturi tube, with the fuel jet at its narrow section. The venturi is surrounded by a vacuum piston. The engine vacuum acts on the piston through a pipe so that the upward movement of the piston takes the fuel along with the air flowing in the venturi.

This system is used to compensate for the variation of the air-fuel ratio during acceleration.

As the throttle valve opens, the air in the venturi flows with higher velocity and due to the Venturi principle creates a larger vacuum which in turn lifts the piston. This allows additional fuel to enter into the venturi tube.

With the closing of the throttle during idling, less fuel is required. The drop in vacuum in the venturi causes the piston to move down, decreasing the fuel supply to the venturi.

The main advantage of this system is that it is the simplest control system which automatically compensates the fuel in the venturi tube according to the airflow rate.

The other systems like air bleeding and fuel bleeding may disturb the mixture by the addition or withdrawal of air and also need additional air bleeds and jets.