The power valve controls mixture of the power circut, which is fuel that goes through the main fuel booster nozzles.
Fuel that goes through the idle circut is governed by the idle feed restriction and idle air bleeds not the power valve.
A power valve with a hole in the diaphram can leak fuel directly into the carb and cause a rich idle mixture that way.
If the vaccum is too low, and you have to open the throttle blades too much to keep it running, the carb will uncover the "transition slot" just above the idle fuel hole in the primary throttle bores.
It can pull additional fuel through these holes, and may run rich from that.
The power valve govererns part throttle cruse mixture when closed, and at full throttle it opens for full rich mix.
The vaccum leak added air but no additional fuel to go with it.
The lower vaccum also causes the carb to mix fuel very poorly.
A good vaccum test guage has a snubber or restricting valve on the inlet to choke the vaccum signal and dampen out the pulsing, this will give you an average vaccum reading, but undampened will indicate vaccum leaks or bad cylinders more.
Check my web page for First Gen Camaro suspension info:
David's Motorsports page
First Gen Suspension Page
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Chev 350
Can Am Vintage Racer
[This message has been edited by davidpozzi (edited 05-04-2001).]