mswilkes65Chevy,
I don't know if your engine has a timing chain or gears...I'm guessing it has a chain. Anything that changes the relationship between ignition #1 and the mark on the pulley will show up as movement in the timing mark...did I say that correctly?
This could be a broken centrifugal advance mechanism, a bad vacuum advance unit OR a worn out timing chain. If you pull and plug any vacuum advance line and manipulate the throttle as you watch the timing marks, a loose chain will usually make the mark move erratically advance and retard...it will move around as the chain goes under tension and then goes slack.
If your engine still has gears, I'll take ten lashes with a wet noodle.
Stuart
