[66-PMD-GMC]

Originally Posted by LostMy65

^thanks guys.

First thing tomorrow, I'm going to check all the ports - including the back.

Searching the forums I found a couple ways to test and find vacuum leaks.
If I can put a rag over the top of the carb, and the engine keeps running, then it's surely a vacuum leak.
But even so, I'm still going to do the suggestion posted in the forums on how to find the leak. That is to spray carb cleaner at one location at a time, if the engine races, then that's where the leak is.
I actually knew about spraying carb cleaner to find leaks, I had just forgot about it. I'm getting old.

Don't forget that you have an "emissions" carburetor, you have most probably yanked all the emission crap off so yes, vacuum leaks are very likely. Also, the motor sat while you were doing your build so it is entirely possible that you also have junk in the fuel bowl and elsewhere inside the carb.
I would suggest using what you have for now and saving your $ getting a period correct carb, a 67 Rochester 4bbl.