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From what I understand, the vacuum normally decreases after some point until it reaches 0 in Hg at WOT. I'm guessing that the power valve is supposed to open up once the vacuum dips below 5 in Hg(?). So the circuit that the power valve is part of operates at high rpm?
I apologize in advance for the rookie questions...

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(1) the intake tract vacuum decreases inversely to the load you put on the eng and gets close to (but not to) zero at WOT (2) PV opens close to the stamped # (5 in this case) on the PV (3) The "power" circuit adds fuel at any rpm when the vac drops below that number (4)1968RR we are all rookies . I'm not a carb guy but I'm wondering if adding the hole in each pri throttle blade mod would raise your vacuum and I think I would add a vac gauge in the dash to keep tabs on how much vac you have at the range you operate in. right now a PV # even lower than 5 may be appropriate. EDIT what is your initial timing at?