When I look down the secondaries and pull the throttle (engine not running), the secondary blades appear to stay shut. I had always equated that with 'vacuum secondaries'.
My trusty '67 Pontiac Service Manual shows that there are actually two sets of valves in the secondaries. At 42 degrees of primary opening, the lower secondaries start to open - they reach full open when the primaries do. The auxiliary valve, located above the secondary, doesn't open until air flow pushes it open. That's why I couldn't see the secondaries moving with the throttle.
