It's part of the emission control system. It reduced the amount of advance in 4th and 5th.

All better! Guess the switch just failed, remove and replace. Again I'm new to these things so I've got a lot to learn.

Thanks for the guidance

It's part of the emission control system. It reduced the amount of advance in 4th and 5th.

Howdy Steve. I spent some time trying to figure out what this vacuum advance switch was on my car and found that it was an emissions feature that actually advances the timing in high gear. My limited understanding of emissions is that retarded timing gives "cleaner" exhaust, maybe that's what they were trying to do.

Attached a copy from the 1976 Emissions Controls section, page EC-9.

The way I understood it, it cuts the advance in 4th (and 5th in the 5 speed) and allows the advance to work in the lower gears. Isn't that what your attachemnent says?