Strange problem with 6-71

I did not know that. I was applying diesel theory. Positive displacement per revolution. now that I think about it , how can you have negative pressure @ idle and positive off idle. You are have a fixed displacement per rev. If you are over or under driven the displacement would still be fixed. Yes? I don't mean to hi-jack your thread for an education. Just trying to understand in effort to help contribute to a potential cause.
Good luck.

Jack is right, at an idle a 6-71 Blown 340 should be and normally has about 12" of vacuum (Intake/"Pressure" side of the system).

Crack the throttle open and it instantly moves from Vacuum to Boost.