Why on the early model 70's evinrude and johnson engine they turn over real slow. I have a strong battery inline and new plugs. I have a johnson engine and don't know what year it is. If I had to quess I think it is a 1970 to 73. The front of the motor cover has power shift # on it and it has a single control for the motor. I put new plug in the motor and it still turn over slow. I can take the plug out and it turns freely. Put the plugs back in and turns slow. I change starter and that didn't work. I look on the transom plate and there no number on there. There is a tag but know number. I put a strong battery inline and that did't work. I need help .