My 40 horse was doing the same thing on my bass tracker. I changed the fuel line and even bought a new plastic tank, and still ran bad. I took it to a mechanic i know and what was causing it to do that was the fuel lines on the carburator itself was getting brittle flaking off in my injectors. He said it would crank and run slow till i gave it more gas and then it would start clogging it up. He replace the lines and cleaned the carburators out. Havent had no troubles since. Runs like an new one. Hope this will help you.