If the hesitation is from an idle, the idle mixture control screw may be out too far.
If you live in an area where deathanol is served at the fuel pump and the hesitation is from an idle, you may need to open up the idle tube in the carburetor.
Also, while you commented that your engine is well-tuned, do not discount the possibility of ignition timing on a hesitation.
And one other possible issue. Since you mentioned that the carb is N.O.S.; Carter used velumoid gaskets (as did Rochester and other carburetor manufacturers during this time period). These gaskets shrink over time. When installing a N.O.S. (new old stock) carburetor manufactured before 1990, ALWAYS disassemble the carb, blow out all passageways (spiders, dirt-daubers, etc.) and replace the gaskets. Then you truly have a new carburetor.
Since you mentioned that you replaced the diaphragm, hopefully you blew out the passages and replaced the gaskets at this time.
Jon.
