I bought my 20hp Yamaha four stroke in 2009 new. When I first got it, my buddy and I could go around 22mph on a rough lake. I just took it in to a mechanic and he replaced the prop and said the carb and everything was fine on it. He said he took it out on the water and it is zipping around now. I took it in because I was lucky to get 8mph on a calm lake. He said the rpm's were around 6k which is apparently where they should be. So, I took it out with my wife and two little kids on a calm lake in northern Wisconsin. We barely got 12mph. I have already done a complete overhaul on the engine. I don't know what else to do. The first couple of years I lived in Milwaukee and no one tells you down there when you first buy them to only use non-ethanol. So now, I only use non-ethanol with marine stabil. I have run seafoam through. I have changed the plugs, the prop. I don't know what else I can do to improve the performance. I understand that I will most likely never get back to the performance of when I first bought it, but if I could at least get it up to about 18mph, I would be happy. Any thoughts? Please, any help is much appreciated.
