Hows your timing at idle look with the vac advance hooked up? You shouldn't have any vacuum advance at idle.

That 750 carb is big for that motor. A hesitation off idle is a fact of life with big carbs on small motors. When you open the throttle fast, it takes time to build vacuum in the venturi. The carb can't pull fuel from the main circuit without a vacuum in the venturi. The solution is to run it richer, it will help the low rpm response but may be too rich up top. Without lots of expensive tools, you've just got to experiment around a little bit with different jet sizes and accelerator pump shots.