Baby it's cold outside.

Thanks to everyone for your suggestions, I believe that I've found the issue.

Only one part was missing. The vacuum hose from the snorkel valve to the underside port of the thermal switch was missing.

After adding the hose, and firing her up, the snorkel valve was completely closeed and pulling air off the manifold. After she was warm the valve was shut and she ran like a top in ~40deg air.

Thanks again.