I have a 1991 28 H.P. Johnson engine and was fishing on a lake. It had been running fine all week, and suddenly, when I went to full trottle, it would just bog and slow down. It still ran fine at about half throttle, but when go to full it would slow down. I could choke it and it would surge somewhat, but would not go full out. It starts fine and runs good up tp half throttle. When I got home, I drained all the fuel in the tank and carb bowl. Looked good, no water or trash. I took the fuel tank screen off and it too was clean. It has new spark plugs and fuel pump. Any suggestions would be appreciated. Thanks