Rebuilt 273 won't idle

Thank you all. I see many of you diagnosed a low vacuum condition and too rich near low speed. That seems exactly right since I see many flame ups and backfires from the carb. I'm going to check tomorrow to see if my secondary is closing. That would explain it.
It's hard to see all of this when you're working alone and can't keep it running. I'll let you know when I get it running.