 As the throttle valve is progressively opened, the manifold vacuum is lessened since there is less restriction of the airflow, reducing the fuel flow through the idle and off-idle circuits.This is when the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle (i.e., as the velocity increases, pressure falls).The venturi increases the air velocity, and this higher speed and thus lower pressure sucks fuel into the airstream through a nozzle or nozzles located in the center of the venturi.