https://www.slantsix.org/forum/viewtopi ... 4&start=45 last post at the bottom of this page seems to indicate a non-operating power enrichment system. Cruise and acceleration under low vacuum are both at the same air fuel ratio. No change

By 1977 even the heavy duty emissions trucks were using the new vacuum dump system for the power valve. Just beginning to dump vacuum

Wide open vacuum is completely dumped the spring is not fighting vacuum full force of a spring is available to open power valve.

Red is older 74 75 models no dump valve ,green 76 all cars and most trucks with dump valve