Mr Driveability may be on the right track with the vacuum advance hose on the manifold vacuum port not the stepped port.
First you should find out what your timing actually is at idle with the vacuum advance disconnected and set it to spec.
If your timing is not advanced enough at idle the fuel air charge may be burning as it exits the exhaust port which will transfer a lot of heat to the water around the port.
If you have the vacuum hose on the stepped port the vacuum advance will not activate until the throttle opens past the idle stage, if you have the hose on manifold pressure port the vacuum advance will activate at idle and the charge will burn before the exhaust valves opens and the engine MAY run cooler.
I have my car running at 19 degrees at idle and it operates at a lower temp then when I have it set to 9 degrees.
Just a thought.