To test: shut off the vacuum. Don't use full throttle.
Rotate the distributor to advance the spark 5°.
Is the idle speed up?
Is the response better?
Is light throttle vacuum higher at the same MPH?
If all are yes, you need a bit more initial (and perhaps some change in your centrifugal total).
If results are inconclusive (not always the same, or too small to be sure), leave it alone.
Test 2:
Shut vacuum off, advance spark 5° and run at WOT with current advance curve over known distance to reach high speed (at least 60).
Does it knock continuously (not just as you nail it)?
If yes, stop and go back 2-3°, and re-try.
Is MPH higher?
If yes, your total is short.
If no change, back off and re-try at +2-3°
If worse, try 2-3° less than stock.
Test 3:
Using best initial spark lead from above, record cruising vacuum at 60 mph.
Advance spark 5°.
Is vacuum higher?
If yes, try another 2-3°.
If yes, back off initial adjustment to best setting, and add test amount to vacuum arm travel.
If vacuum is higher but it knocks on light tip-in, adjust the spring in the vacuum can stronger (turn the screw out, record how many turns).
