[Tinker]

I'd guess. and it's a guess. Carburetor needs attention or an engine re-ring or build is in your future. Low compression consistent means rings also. If it was erratic compression maybe valves.

Testing vacuum and fuel in the garage is fine. But underload is always different.

Vacuum at idle should be in the 18-20 range. Not 12. But if the air/mix doesn't do anything to increase, or more importantly lower it. Carb or a vacuum leak. I hope you get it figured out!!!