Adj vac advance on factory dist.

[TT5.9mag]

Adjusting the spring either compresses it or reduces the tension. If the Allen screw is fully CW, less vacuum is needed to move the actuator compared to when the screw is would a few turns CCW. So the Allen screw affects spring tension, which in turn affects the amount of vac reqd to move the actuator.

Exactly. And no matter how you try to explain it, someone always thinks the opposite. That tightening the adjuster screw makes it take more vacuum to work. It's just like you said.

I think what gives people the most trouble is that it works backwards from conventional thinking. Tighten the adjuster, less spring pressure, less vacuum needed to start movement. Loosen the adjuster, more spring pressure, more vacuum needed to start movement. The picture posted by @halifaxhops makes it easier to visualize how the spring cup moves with the adjuster screw.

 

[crackedback]

The rate of timing change isn't the spring. Correct the spring rate doesn't change.

Can has 16*, Max advance with vac at 18inches. If you adjust preload so adv begins at 10" or at 14", the rate advance is added is different across those values. One is 2*/inch the other is 4*

 

[Woody500]

There is another kind of "adjustable" advance, which came on the dual point distributors (the ones with the hex on the nipple). You unscrewed the hex and added or subtracted washers to adjust the advance. I have never seen any documentation on how to do this systematically, or which washer pack came with which engine combo. But at one time I had 6 dual point small block distributors, and different ones had different washer combinations — for example, Commando 273 with or without California Clean Air Package.

View attachment 1716450627

That's a Prest-o-Lite distributor or Aut-o-Lite(not Ford) depending on year. Probably tag says Chrysler. Totally different than Standard Chrysler, some which have a hex on adcance can, see post #4, it has one.
I didn't know they could be adjusted that way.
Not sure what it was used in other than a Hemi from factory.
Please school me.

 

[Vinny]

You added advance and the pinging went away?

That is my question too.

 

[slantsixdan]

If I recall, the advance adjustment was mentioned on factory service manuals were high altitude tuning was the topic.

Believe not. That screw in the vacuum advance was not meant for field-adjustments; it's not like the idle mixture and speed screws on the carburetor.

The FSMs I've always read say this, and only this, on the subject:

____________________
Vacuum Advance Curve
If only the vacuum advance curve is to be checked, connect tester vacuum pump hose to distributor vacuum advance unit and perform operations 1 through 3
under "Centrifugal Advance Curve", then proceed as follows:

(1) Turn tester vacuum pump "ON". Adjust vacuum pump regulator to vacuum test specifications. See "Specifications" and observe arrow flashes on tester degree ring to determine degrees of advance.

(2) If vacuum advance is below or above specifications, replace the vacuum advance unit. Retest vacuum advance curve.
____________________

It's possible my memory's faulty and my spot-checks missed a year where the vacuum advance adjustment was mentioned, but I think probably not.

 

[Mattax]

It's possible my memory's faulty and my spot-checks missed a year where the vacuum advance adjustment was mentioned, but I think probably not.

Not an FSM, But the 1951 MTSC in my How To linked earlier explains what to do with the washer type advances.

 

[slantsixdan]

As to why factory advance pods started having adjustment screws for '72 when a bunch of other changes were also made to the distributor design details: I wasn't there, but my best edumacated guess is that "more or less kinda close to the nominal spec" was no longer sufficiently tight tolerance — not when automakers started being legally required to certify that their cars would meet the applicable emissions standards. That screw was almost certainly intended for use at the end of the distributor production line.