Please learn me about the adjustable vacuum advance cannister on the HEI distributor.
I thought this unit was for adjusting how much ignition to be added between idle and app 3000 rpm's, but that's maybe wrong....?

I have turned the thin 3/32" allen screw in and out several times now, and probably used to much force on it, so now it feels like it's destroyed.
The difference between idle and 3000 rpm's was 20 degrees no matter what position the allen screw was in.

Engine is a 350 Chevy with modified Vortec heads and a .500" lift cam, but intake vacuum is close to standard.

 

Depending on where your vacuum source is your canister will only add timing at idle, light cruise or zero throttle with manifold vacuum if ported you get no advance at idle. As you open the throttle the vacuum goes away what the adjustable is generally how much advance at a set vacuum point.

http://chevellestuff.net/tech/articl...r_manifold.htm
http://www.superchevy.com/how-to/add...nition-timing/

 

Vac adv is all about cruise.

Depending on where you have it hooked up... if it's ported vacuum, the place the vacuum comes from is above the throttles at idle, and thus it won't be active at idle. BUTT when the throttles are opened somewhat, and vac is high (meaning, relatively high speed but little/no engine load), the vac adv gets vac, which makes it adv. The point being, the more advance it is given at cruise, the leaner the mixture can be (the less fuel it has to consume), and still do its job.

Cars will often run better with it hooked up to constant vacuum as opposed to ported. Try both, if available, and see what your car likes better.

So, what the screw does is, it sets the amount of vacuum required to make this change happen. You won't notice an effect at idle. Easiest way to set it is, keep backing it off (reducing the amount of vacuum required to activate it) until you get pinging when you just barely open the throttles at a 100 km/hr steady-state level-ground cruise; then crank it back in (require progressively higher vacuum to operate it) until the pinging goes away under those conditions; then crank it in a little bit more, just as insurance.

It is different from what you described initially, which is "centrifugal" advance. Or more accurately, "lack of centrifugal retard". Virtually ALL engines running on gasoline require very similar advance at their max torque / max HP RPMs; but require FAR less at lower speeds. Unfortunately engine calibration settings don't work that way, in spite of the theory.... the real world gets in the way and makes it have to work differently. The centrifugal system accomplishes this via the springs & weights, and is entirely independent of vacuum.

 

Inside of that canister is a spring, a screw and a nut, and the diaphragm. Turning the screw with the Allen key adjusts the tension on the spring - controlling the rate of vacuum advance. The end of the screw is supposed to be peened so that it stops when you reach the end of the threads. Some are not peened really well, and if forced a little too hard, totally unscrew. If that is what happened in your case, then the canister is now trashed.

 

Thank you Sofa. I started to write all that, but just ran out of steam.