When you say new carb; 1) new, right from the box and you put it on, 2) a rebuilt ( you or store bought), 3) New but on the shelf awhile.
I got a street avenger Holley last year from a guy locally who ran it briefly on his wife's cuda S. Engine was questionable so the carb came off after a few weeks and put on a shelf in late 2015. I grabbed it in early summer. I was a little carbon-ed up but not back fired fried. Guy complained the secondaries wouldn't open (Vacuum actuated). He had the quick change spring kit and the lightest spring in there. I ran the carb just as it was sold to me and tested the secondaries with the paperclip - They worked just fine/ a little too early, so I put a stiffer spring. The week of preps for Mopar day @ Englishtown New Jersey in August, I noticed fuel like you described. The Vac ports at the bottom were filled with fuel. Vac adv port was fine and it is the only one I use. The brake Vac and Crank vent go right to my manifold. This gas build up was from some carbon. Mind, I never sprayed the carb or cleaned it. I just bolted it on. Now, folks can throw rocks at me, but I figured a local old school mopar guy who sells me a carb from his house is not going to lie about the carb, so it was a risk I was willing to take. I run with multiple fire extinguishers always. I store the car indoors, but roll it out immediately - so not to burn down the house. I mean treat your old car like it will catch fire and stay prepared! I've ranted that before.
So this fuel buildup continued.
As the fall car shows came to a close, I wanted to just give a good spray of carb cleaner. The car was warmed up and out of the garage. A few good sprays, start up and a few more. Went through a good amount of cleaner. Now the fuel build up aint there and it does have a crisper response, but the oil is now contaminated with the cleaner and carbon wash. The carb worked well, like you describe even with the fuel build up. So well, I famously broke an axle at the track with that "mildly dirty" Carb.
So that is my take on the Carb.
The other issue you may have is timing. If fuel is bouncing off closed intake valves, especially in rich mode/ Choke closed it can bounce back up through the carb. My brother's 73 Fury with the 360 did this. That was a a timing chain / gear issue. ( My old man was doing burnout's with it while my brother out cold after final exams).
What is the car doing while driving? Does it sputter, have decent power? What other symptoms do you have?
Give us some motor details: Slant6, 318, 360? 1 BBL,2BBL, 4 BBL? what carb brand?
Stick or Auto?
Lean burn Controlled?
Give with as much detail as you can.
