I second the fuel line possibility suggestion. I have the exact year of outboard on my pontoon boat. My main problem was the engine would push the boat fine for about 300 yards at full speed then bog down and only run smoothly at half throttle or less. I rebuilt the fuel pump and replaced the fuel filter. It turned out to be the fuel line from the built-in tank on the pontoon boat. It was blocked by varnish-looking strands in the line. To easily test it, put some fuel in a separate portable tank and use a different fuel line to test the boat. Having done the test and replacing my fuel line, it has worked great ever since.