I wonder if it's possible the carburetor's choke cam was on one of the steps causing the primaries to be open a little too far? SOMETHING is causing you to get vacuum at idle on that vacuum advance port.
Well, what I Didn't Do, was check w/ the vac gauge, just plugged the can back in. Wonder if the springs in the mechanical advance are just super light? I have one o them might vac gimmicks too, garage sale deal... no idea how to really use it