Needing some help diagnosing a problem with my motor. As the title says I have a 1960 Johnson 18hp seahorse.. It was running like a champ and was getting about 18 to 19mph out of it then it seemed to lose a little bit of power, I checked the speed and I was down to about 14 to 15 mph. It ran like this for about 5 minutes then lost almost all the power, was doen to 5mph at best. I tried adjusting the high speed idle but it didn't seem to help. When I have it in neutral with the throttle wide open it seems to be running fine, by that I mean the rpms seem to be there, it's screamin. But when I put in gear and throttle it up.. nothing 3 to 4mph at best. I cleaned the plugs and regapped them tried new gas, nothing seems to help.