[vettestr]

I can think of several reasons for a low manifold vac reading. Here are a couple of things to do or check in search of problem. I doubt timing has jumped a tooth but timing could be set wrong or wrong point dwell or bad electronics in MSD box.

A= Advance initial timing until maximum manifold vac. is achieved and then check with timing gun to verify dampener has not spun on hub. B= Bad or stuck PCV valve will drop manifold vac reading. C= Adjust carb idle screws to set air/fuel for highest vac reading and remember you should be able to kill engine by closing either side or you have a vac leak. A pyrometer will help identify a lean cylinder. Take a reading about an inch from the head/header flange on each tube. Hope this rings a bell for you.