If it's having a hard time running at idle it may not be the fuel pump. It doesn't take much pump to keep it idleing. Of course make sure the bowles are full. I would also check the distributor cap and rotor button. Had that happen to me this summer.
I would pull the carb apart and check everything really good.
and I agree with
More info? What kind of fuel pump? Carb? If the only thing that was changed was the fuel pump then what may have caused this problem. Debris in the fuel line? did you use teflon tape on the fitting? Etc.