Trouble with my ignition timing

my5thmopar said

Turned distributor until I got max vacuum at 20". I ended up at 30 degrees advance. I have checked balancer with piston stop. I also checked it during assembly with a dial indicator. The balancer is less than 1 degree off. The engine turns over with no kick back and idles well. When run the rpms up 2500-3k the advance increases another 30 degree to about 60 at balancer.

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Intriguing. The only check you haven't done is verify that your timing light is accurate. I assume you are using one with an advance dial. I have read that many are notoriously inaccurate. One check you might make is to mark 30 deg advance on the balancer and check with a basic non-advance lamp, or could do the same with timing tape.

My 69 Dart was similar. It idled best at ~30 deg advance, and never idled well, slowing way down and wanting to die when you shifted to drive. I also verified the mark with the head off. If I set it at 5 deg BTDC, it would like to speed up in neutral where the mech advance gave it the high timing it liked. It finally idled correctly after I installed a 4th Holley carburetor, so I suspect it was running too lean with the others. Consider installing an O2 sensor. Otherwise, you are always flying blind with a carburetor.