Yes, the vacuum advance diaphragm on my engine was shot, and hence the source of the vacuum leak. It idled well, since vacuum advance hose carries no vacuum at idle.

Currently the hose going to the distributor is plugged -- meaning no vacuum advance, so the engine is running retarded under light loads. When I floor it, it runs perfect. After it warms up, even part throttle works great.

Does anyone know if there is an adjustable advance unit available for the stock A12 distributor?

The static timing is right on: I advanced the timing until it pinged a bit, then backed it off a degree.