Erratic timing during vacuum advance

Duster Daddy,

One of the things that I have learned recently myself about Vac Advance Cans is that there are many available. One key feature to consider when selecting the right Can is the amount of vacuum present at idle which is dependent on your camshaft. The proper way to select a Can is to be sure it is "all in" 2 inches of vacuum below your camshaft at idle. If they are one in the same or higher than the camshaft the Can will "hunt" and provide erratic timing and idle characteristics. Basically, the plunger will oscillate. My Comp XE274 idles with about 11-12 inches of vacuum so I selected a Can that is all in by 9-10 inches of vacuum and only puts out 14 degree of advance to keep pinging down and allow for more mechanical timing too. My timing tape is now rock solid under the light and the idle very stable. I got mine through AutoZone but Echlin (Napa) and a whole lot more sell these for 12 bucks or so. I have a GM HEI Distributor FYI. I have a link I can provide if you are interested at home that is very informative on vacuum, vacuum sources and vac advance.

FYI,
Marion