MAKE SHURE THAT THE CARRB IS THE CORRECT
APPLICTION YOU NEED...CFM.rebuilt carbs need adjustment,after being on the dyno rebuilt.
IE:check fuel nozzles size. if it is correct or need bigger ,try adjust idle mix
and no vac or fuel or gasket leaks,make shure the other adjustments are aces choke,flap
rpm ,secondaries,binding linkages.
you said it`s hard to start cold then you said it starts ok then goes for a barf
after 20 mins.you have to baby it to run along.
if set up is ok then it
could be internal problem
in carb ie sticks - float needle valve,or blocked secondary fuel nozzles or saturated or sunk float,or metering rod or size problem
or cracked at base somewhere does it backfire or fuel filter in upside down or
plugged or .go to town on it .sound like you should re investigate visually or
send it back if it was rebuilt.