At least 30 percent of our work here is fixing issues with freshly "rebuilt" carburetors. Could be just about anything, and in most cases I can nail down the problem(s) in less time than it took to type this.
Yesterday I went completely thru a 1970 Buick 455 carb "fresh" from a rebuilder. It was a MESS, full or "low end" components, including a black rubber cup on the old accl pump that had already swelled up and binding in the pump bore.
Float was WAY too low, throttle plates not fully seating when closed, or fully open to 90 degrees at WOT.
Wrong needle/seat assembly, mangled up used jets, wrong power piston spring, and not sealing at the top gasket.
We went completely back thru it, and it ran flawlessly on our test engine.....Cliff