Post #88 & the reason for the change from MVA to PVA.
It was for emission purposes, as I stated in a prior post.
You will find the info HERE:
www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
You may have to type in the full address. I believe the article was written by GM engineer Lars Grimsrud.
Here is the relevant comment: ' Now to the widely misunderstood manifold v ported vacuum aberration. After 30-40 yrs of controlling vacuum advance with full manifold vacuum,
along came emissions requirements.........One of these band aids was "ported spark". This meant the vac adv was inoperative at idle [ retarding spark timing from its optimum value ]....