"Rebuilt" is a broad term. Could be a multitude of things wrong, heavy float, float set incorrectly, bent metering rod hanger (common problem), power piston stuck, jets drilled out to a larger size, etc, etc. The jets and metering rods could also be worn out, as they slide together for many years and both items wear considerably in some units.
The best course of action would be to install one of our SR kits, with new primary jets and metering rods. I'd replace the power piston spring, and install a new secondary plastic cam and spring as well.....Cliff