The Stovebolt Site Forums The Parking Lot Big Bolts 348 diagnosis

Inside of the distributor is a centrifugal valve, it opens and closes based on engine/distributor speed. That valve is a vacuum bleed and could be gummed up. It is underneath the electrical section, mine has a strap that covers a window that is the access for governor speed adjustment, the strap should have a paper gasket to prevent vacuum leaks.