Is the initial timing too high? If there's a combination of excessive initial advance and the vacuum advance is connected to manifold vacuum, this could be a cause.
Is the mechanical advance working correctly, with the springs able to positively return the weights to closed position at idle? If the springs are too weak, or the advance plate is sticking, there will be too much advance.
Try disconnecting the vacuum advance and plug the hose to see if the idle drops.
What is the condition of the PCV valve and hose and connections? The PCV hose will become hard as a rock over time and will leak at the valve and/or carb end, this is a massive vacuum leak, as well as if the PCV valve itself is bad.