Adjustable Vacuum Advance

VA is best used with manifold vac [ MVA ] , NOT the useless, emissions related, ported VA. This especially true for these conditions: low[er] CRs, larger cams, big sluggish ports, single plane intakes. All of these require more lead time for the ign to fully combust at idle & cruise. For modified engines, an adjustable VA is ESSENTIAL. One of the reasons VA gets a bad rap is folks trying to get a non-adj VA unit to work on an engine with reduced idle vacuum, resulting in idle timing varying & being erratic. Not the fault of the VA concept, fault of the idiot doing the job.....

Beware also that some adj VA units [ like the Crane ] have a sleeve inside the spring that limits the TOTAL VA available. Un-crimp the can & remove the sleeve or find a VA that still has full plunger movement when adjusted fully CW or CCW.
A big cammed engine may need as much as 50* [ yes 50*!! ] at idle for best idle quality, highest vacuum, best tip in response & coolest running at idle. One of the few things Mother Mopar got wrong was NOT to use MVA.

To set up: turn Allen Key [AK] fully CW [ softest spring setting ]. AK is usually 3/32".

[1] Warm up engine, chock wheels, put in D if auto. Disconnect & plug VA.
[2] Loosen dist clamp, & starting at around 10* BTDC, slowly turn dist to advance the timing. Idle speed will increase & get smoother; keep advancing timing until the highest idle rpm is achieved. Toggle dist to make sure this is the highest rpm. I call this the 'sweetspot'.
[3] Now check what your timing is. Say it is 34*. And you are using 14* initial [ static ] timing. The VA needs to add the remaining 20*. This the TOTAL timing added by VA. Most adj units will provide up to 30*.
[4] Make/use a stop to limit the VA to 20*.

Vac Adv RATE:
[1] Connect VA. Check timing, in gear, engine running. Turn AK 2 turns CCW & re-check timing. Keep doing this until timing drops or becomes unsteady. Then turn AK CW one turn at a time until the timing is steady. Then go another 2 turns CW.
[2] Job done, VA is adjusted.

Any change to the engine will require the adjustment to be done again.
This procedure works every time, not sometimes.