Why does my 60 hp lose power when accelerate to full power?

I hav a 17' 60 hp boat and wen im in te water and push it to full throttle it loses power and sounds like its going to shut off and sometime it does and sometimes it sounds like it and when i bring it back down to idle speed it stays on but huffs and puffs changed gas line,gas filter, and spark plug which they had a car spark plug in it need an answer to y when i push it to full throttle it wanna shut off and goes like 2 miles an hour when on full throttle.