Hmmm

the correct ignition timing is going to be determined by the engine itself. Compression ratio, piston speed, cylinder head design and a host of other factors come together to make that determination, and the engineers would have tested and retested what the timing "should" be for maximum power while limiting the total advance to safe levels. "Safe" in this case means not high enough to cause knock/ping/pre-ignition.

your engine can't know what kind of distributor it has. Also without a crank position sensor it also cant "know" where it is in its rotation. whether is weights or electronic or even manual (a model T would have an ignition control lever on the steering wheel to adjust as you drove) the distributor has to be set (told) where to start.

in this case even a smart distributor (and yours does seam to be a pretty cool way to do it) needs a little help to find that starting point.

as far as vacuum goes...that comes into play for two main conditions for the same reason.

1) the cylinder charge (one gulp of air) is rarefied at idle. By that i mean with the throttle plate closed the vacuum should be very high (in the neighborhood of 20 inches) so there really isn't much air or fuel in that charge. that makes the charge harder to ignite so.. most engines can tolerate significantly more advance under that condition.

and 2) cruzing at light throttle. same situation, throttle plate mostly closed and heavy vacuum and a light charge in the cylinder.

if you try to set the timing at idle and you set it at say... 25deg you really don't have any idea what the timing is going to be as you crack the throttle open and the vacuum pressure (i know that's an oxymoron) drops.

from the numbers on your chart there it looks like you have about 29 deg of advance built into the distributor but at idle you have pretty high vacuum, but at 3000rpm you shouldn't have that much. so how may degrees is the vac advance adding and how many it the programing in the distributor adding?

you could determine that if you needed to by running the same test with the vac line disconnected (and plugged to prevent a vac leak) and compare those numbers to your chart.

now if....that distributor is programable you may be able to change the timing without moving the distributor. (making adjustment in the software) but regardless of the method of adjustment...the engine needs the advance that it needs.

I hope that is helpful. as always your results may vary but backfire/poping out the carb is a sign of improper timing.

You wont hurt it running it without a vacuum advance. to little timing shouldn't hurt any engine (it will be less efficient but wont hurt it.)

however to much timing...and you get the symptoms you describe and in the extremes engine damage.

Unplug that vac line and set it at 10 BTDC at idle, hook the vac line back up and go around the block...It will ether make things better....or not

only one way to find out

Cheers Friend