No full throttle?

I have a 2003 50 horse 4 stroke merc . I replaced full pump recently. Problem is motor starts every time , and will run all day , but when I try to go more than 1/2 throttle it just Boggs down and actually goes slower than at 1/2 throttle. The boat is a 16 ft sea nymph and would normally get on plane but now it won't. I got this from my father in law who had it sitting in a barn for several years. It didn't run until I replaced the fuel pump. I've rode in this boat in the past and know it's not right now...any ideas would be appreciated...