[Kurt in NJ]

It's not a vacuum advance like more modern cars have, it is a vacuum relased retard----called vacuum brake, under conditions of low vacuum a piston rubs on a wheel that limits the advance-----if the piston is stuck up the advance could be faster, the piston stuck down and rubbing hard you won't get full advance
if the polished surface has rust damage on part of it and a different friction the advance can be erratic during low vacuum, and the leather puck on the piston gets chewed up ----all the cars I had with the adjusting screw backed out most all the way it never pinged on fresh gasoline (I don't know why it changed type in mid typing)