You need vacuum to shut the fuel off. Not enough vacuum to a large power valve and the fuel is just poring in. divide the vacuum in half leans the charge off idle and through the mid range. I found that when using 1/2 the number power valve of vacuum my EGT's on the guages were perfect through the quarter. The motor was like a off and on switch. This is right off idle foot braking. With out doing half it would stumble.
No fuel moves through the power valve until you get up on the booster. It's impossible. Like I said, Google Mark Whitner and power valve test or something like that, and he PROVES that the PV doesn't affect idle, unless the valve itself is damaged.