Agreed with Mark about the power valve, and for exactly the reason he states. Ruptured = bad, almost non-running at idle since liquid fuel will be sucked straight into the intake manifold under all running conditions. 'Open' at idle (vacuum rating too high relative to the vacuum the engine produces at idle) = no difference in curb idle quality.
1990 454SS pickup. Sold, but I think she found herself a good home.
78 Malibu. Inherited with only 35K on the odo! Mild 383 with Weiand 142 blower on top. Mid-high 12s on street rubber, bottom 12s on DRs with the trans left to shift for itself. Runs so good I may never drive it again.
"Last remaining QJet tuner on planet Earth!"