carb question

buying a carb 750 double pump is there much difference in perf , with a carb that has top cut for better air than one thats not ? thanks my 750 looks great but will not run the motor ,hit the gas pedal squiters squit gas , but wont run, tries to run if you pump the **** out of it, put another carb on fires right up, been apart several times cleaned ,checked cant find trouble new needle and seats,set gas level,getting 6 to 7 lbs pressure , not flooding ??????????