This is true, and a very good point. However Teammerritt indicated that the car ran fine and that the problem seemed to get better if he advanced the timming. If it were running lean, advancing the timming would have made the car have much less power, and spark knock would be terrible.
If the vacuum and/or mechanical advance are not working, the car will seem to run fine, it will however run hotter and use much more gas. The exhaust gases will still be burning when they exit the engine, thus the exhaust manifolds will see a blow tourch effect.
84 500 SEL (307,xxx miles)