How can a vacuum leak cause valve timing to retard?

Valve timing does change slightly with timing chain wear, and IIRC it causes timing to retard slightly, and I recall that the wear limit on my M103 is about 1.5 degrees. The V-8s are probably more due to the much longer chain, but 8 degrees seems like a helluva lot.

Do you know the age/mileage of the chain?

Duke