Is the B-body 428 timed the same way as the 389 and 400? I have one here that is plumbed to manifold vac and falls on it's face only when the 4bbl opens. Should it be ported? All tune-up parts look good.
Should be, I don't see why not. Sounds more like a carb problem. Could be the accelerator pump ( Quadrajet Accelerator pump Page
) is not giving the carb a shot of fuel when you open it up. This will cause the car to go flat. Next, your secondaries may be opening too quickly, sucking in a big gulp of air before developing enough air velocity to draw in the needed gas to go with the big gulp of air. Does it fall flat and then eventually pick back up? Does it do better if you gradually ease into the carb as RPM's increase and open it fully up as opposed to slapping the gas pedal to the floor to pop the secondaries open?