Join Date: Sep 2000
Location: Washington, Michigan USA
Look at the basics first - when timing jumps around at idle, it's not necessarily the centrifugal advance mechanism, as it doesn't come into play until well over idle rpm; however, could be a vacuum leak between the carb and the vacuum advance can or a leaky can. Disconnect the vacuum advance (plugging the hose) and see if timing still moves around - if not, you found the problem; if it still moves around, chances are it's worn distributor shaft bushings and/or excess vertical play between the distributor gear and the bottom of the housing. I'd bet on the latter - see it all the time on 30-year-old cars.
'69 Z28 Fathom Green