80's 40hp Mariner

[Ctuttle]

The motor starts great, idles great, but as soon as I get above 1/2 throttle it has no power and starts running rough. Any input would be great.

 

[Texasmark]

Lack of fuel would be my first guess. 2 quick causes would be fuel pump isn't pumping enough fuel or filters are clogged.....then there could be some age problems with the fuel lines. Squeezing the bulb would answer the question of fuel pump failure. Visible inspection of hoses would answer that question. Replacing your fuel filter(s) would answer that question.....and then there could be a tank vent problem and removing the fill cap would answer that question.