fuel evaporating in carb?

Yes you are experiencing the same as the rest of us "carb" users...today's fuel evaporates very quickly. Along with OEM type mechanical pump, I have added electric fuel pumps to help prime the carb so after the car sits awhile, excessive cranking is not required to fill the float bowls. Because of todays fuel, have had to change/tweek jetting to have a smoother running engine.
For electric priming pump the best for me so far has been Mallory, has internal bypass so no regulator is needed as stated on the tech sheet with the pump. Also the design allows easy pull thru so highways speeds are good with just the mechanical pump, or can use both pumps. I keep the electric as a back up on long highway cruises.