Joined: Nov 2016
That vacuum switch was part of the early pollution control stuff. It is connected to the distributor's vacuum advance on the 'middle' plug, to the carburetor (ported vacuum) at the top, and straight manifold vacuum at the bottom.
The idea is to run off ported vacuum (zero vacuum advance at idle) which retards the timing. That dumps still-burning fuel into the exhaust, raising temps in the manifolds and theoretically letting the air-injection (thermactor) system work efficiently. When the engine coolant is above a certain temperature (210? Can't remember for sure.) the switch cuts over to feed the distributor manifold vacuum, which raises the engine timing, helping it run cooler and spin the engine-driven fan just a bit faster. I am not certain, but I think perhaps even the non-Thermactor cars got this switch, because the lower cylinder pressure from the retarded timing helps prevent nitrous oxide formation during idle, even though it really raised hydrocarbon pollutants and CO (carbon monoxide) because of the poor burn quality.
While there seems to be endless debate over ported (or timed) vacuum vs. Manifold vacuum, I feel strongly that these cars run cleaner, better, and cooler on straight manifold vacuum anyway. There are a lot of heated discussions in other posts that you can find, but I will refrain from getting long-winded here. =)
I smile a lot. It makes people wonder what I'm up to...