Waveski, the Q-Jet secondaries have a two step actuation. When the primary throttle valve (this is directed by your throttle peddle) reaches approximately 3/4-7/8 of its full opening, the secondary throttle valve is actuated and linked to the primaries and will open progressively. Fuel will not be pulled into the secondary bores until the engine can produce enough vacuum to overcome a second "air valve" which is above the throttle plate. The air valve is spring loaded to stay closed until the vacuum level is sufficient to open it, when it opens, fuel is pulled into the secondary bores and ingested by the engine. The secondary air valve spring is adjustable and can be tailored to the demand of the engine. The Q-jet has massive secondary bores, it also has metering rods that can be changed and/or adjusted to meet the engine demand, the spec for the more common Q-Jet is 750 CFM, but this is directly dependent on the volume of air that the engine can actually handle. As Carbking has stated, the primary side of the carb is rated at 150 CFM and that is where you spend 99% of the time, when you need horsepower for going up a steep hill or passing another vehicle, you push the "GO" peddle to the floor and the secondaries will feed the engine with whatever it can handle.
