Disconnect the vacuum hose of the throttle actuator diaphragm assembly (A) from the carburetor body and connect a vacuum pump to it.
Create a vacuum of 100 mm Hg. and fully open the first throttle so that the second throttle is fully open (for some models - 25-33% of the full stroke.
If the vacuum does not hold, replace the second flapper diaphragm assembly. If the vacuum holds but the second throttle does not work, check the throttle linkage.