I've had issues with ignition timing stability at idle due to sticky or loose mechanical advance components in the distributor. Lighter springs on the flyweights seem to make it worse. I like to use the heavy factory weights rather than the light ones found in kits. Weight shape where it contacts the cam and the cam shape can be modified by welding or grinding to manipulate the ignition curve.
Yeah, I'm using the heavy stock weights with the Accel springs (although I think the heavy Accel spring is the same as the stock springs). I did have a sticky HEI and took it apart to clean up and apply motor oil to the lower shaft and new bearing grease to the top shaft. After doing that it snapped right back when twisted. That was about 5 years ago, so it's possible it has gotten stickier since then....I should probably do it again.
I was doing some work on the choke yesterday and set the timing and idle again. This time I set it at 10* initial advance w/o vac adv, 15* @ 800rpm with vac adv, and when I had it on the choke's fast idle cam at 1500rpm it was running 23* advance with the mechanical.
