[olddog]

I recall you saying the vacuum is a bit low at idle. For vacuum advance to work correctly, it has to change the timing over the range that your cam/engine operates in. More importantly, if you use manifold vacuum (as opposed to ported vacuum), the vacuum advance must be sized to pull in all its advance (hard against the stop) about 2 "Hg less than the engine idles at. If not, the timing will be bobbling around at idle, and the rpm will jump all over. Timing has a very fast acting, major impact on torque and thus idle speed.

A GM engineer named Lars Grimsrud wrote one of the best write ups, I have seen. If memory serves, it was on a Corvette forum. I believe I found a link to that from this site. If you are a do it yourself guy, do some searches and educate yourself, before you start purchasing. If you cannot find it let me know and I will try too.

Good luck with whichever direction you take.