Re: 1975 Mercruiser 140hp
I am guessing the boat sat a while with fuel in the tank.
The new ethanol fuel acts like a solvent to the years worth of gunk in the fuel tank. When you had the boat out running it, all that debris was loosened and found it's way to the fuel filter(s).
Full throttle was asking for more fuel than the clogged filter could pass, so it ran rough. Backing off the throttle got the engine to a point where it was not asking for more fuel than the clogged filter would pass. After running it for a full day, though, the filter got more and more clogged to the point that it cannot pass enough fuel for any throttle position.
I had this same problem with my boat when I first put it in the water last fall.
If you don't have an inline fuel/water separating filter, you should get one. It's a bigger filter and it is a lot easier to clean.
Your fuel pump has a filter in it's housing. A fuel filter inside the fuel pump was a surprise to me. There is also a strainer inside the inlet to the carb.
Make sure you have all clean filters and then try again. The inline water separating fuel filter spins on like an oil filter. REALLY easy to change out. Install one, have extras on hand. If it runs good with clean filters, then the problem is with garbage in the fuel system.
