One of the real advantages of the QJ carb is the fact that the primary barrels have not one, but THREE venturis. That allows gasoline to begin flowing into the high speed circuit just above idle speed, and as the air velocity increases, the second and third stages of the venturi stack start adding fuel to the airflow. In simple terms, each primary barrel becomes not one, but three carburetors, operating at different airflows. There's not a single venturi in the world that can offer such an accurate level of fuel metering. Then the secondary barrels come into play once the airflow in the primaries maxes out- - - -and those barn doors are on a mechanical linkage- - - -no need for vacuum to open them up. The airflow (and fuel metering) in the secondaries is controlled by a spring loaded flap that resembles a choke plate at the top of the carb- - - -preventing any possibility of over-carburetion. BTW- - - -that air valve has an adjustable torsion spring, so its rate of opening can be fine tuned for a specific engine's needs- - -like camshaft changes or other non=stock equipment like a single plane intake manifold, etc. The Carter AVS and thermo-quad carbs have a similar, but not identical design.
Jerry
