The orange TVS (thermal vacuum switch) is part of the trapped vacuum spark control. With it is also a 4 port check valve.
They way it works is this: When the engine is cold, manifold vacuum and ported vacuum through the check valve and the thermal valve. The strongest vacuum from either source directed to the distributor vacuum advance.
The check valve holds this vacuum on the distributor as the engine begins to warm up. Not to full heat, but just enough to prevent cold stalls.
There is also a cindered iron bleed on the TVS. This will allow the vacuum to bleed off should the engine stall. If that bleed breaks off, then you could have no vacuum to the distributor.
Borrowed pictures:
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The second picture shows the bleed facing. Should have a small foam filter inside the 4 prongs in front of the bleed.
If any part of the plastic TVS is broken, you will not have any vacuum advance.
One fix, without the valve is this: Run a vacuum hose to the ported vacuum port on the carb straight to the distributor advance. Advance the initial timing just a couple degrees.
