[davepl]

Here it comes:

If your vacuum advance is plugged into manifold vacuum the extra ignition lead can cause run-on until the engine slows enough that it's not producing enough vacuum to advance anymore.

If it's not that you likely just have too much initial timing. Sometimes people crank the timing too far ahead trying to get a big cam to idle well.

A less common cause would be carbon buildup in the cylinders increasing compression. You could check that with a compression tester (it should be around 150psi, if you're seeing >180, consider it).