Black soot out of exhaust pipe

You're most likely running rich. You said rebuilt, is this a rebuilt original carb or a rebuilt remanufactured carb? Also is it a Holley 1920? I once had a remanufcatured Holley 1920 and one of the retainer rings in the power valve came out and the check ball was just rolling around in the float bowl. Every morning I would have a ton of black soot come out the tail pipe. You could even see a line of it down my driveway. I reseated the ring with the check ball inside and figured that was causing my rich problem since there was no check ball to stop the gas from flowing. After putting it together I had the same problem. Turns out it was just a junk carb (remanufactured).