Rebuilt top end of engine now carb idles much higher?Please help

I have to apologize in that I’m trying to learn as I go along. Sometimes I may need things explained to me in more detail to fully understand. Yes I tried to back off the idle screw to lower the idling rpm. It seems to only adjust down to where it is at now. I loosened the rear bracket holding the throttle cable to see if this was limiting the low idle. I also tried to push the idle setting lower with my hand and it is being limited at the bottom. What would make the engine idle higher after reassembly? I didn’t change any of the carburetor idle settings? Would a vacuum gauge allow me to determine if there was a vacuum leak. I don’t know what a dwell tach is or what it is used for?