I have a '71 25 horse Johnson and it's giving me fits. I've replaced the impeller, carb, head gasket, fuel pump and fuel lines and even bought a new plastic tank to assure there would be no trash. (And yes, the fuel is brand new and properly mixed.) Problem is, it won't "open up" and insists on running at no more than low rpm which only gives me a top speed of about 5 mph. It'll run all day like this no matter what I do to it or how hard I twist the handle. The weird thing is if I put it in reverse and give it some gas then the motor will rev up to full normal speed like nothings' wrong with it. What gives? By the way, it won't start at all unless I put it in forward and twist the trottle open as far as it will go.