I've tried that too (backing off the primary to almost no t-slot)... didn't seem to make much difference. How big are your blade holes?

How come you only have 6" idle vacuum with a smaller cam than my 451? Idling real slow and lumpy? Or a flat-tappet (mine is a mushroom)?
What's special about the FV178 valve?

A major difference I see is that your MAB is much smaller (and your emulsion stack is smaller too)... .028 MAB is the next thing I'm planning to try.
Raining all through tonight, so no road testing today!