I have a 76 15HP Evinrude pushing an 80s something Sylvan jon boat. I cant get the boat to plane. Motor runs excellent. I can feel it start to push the boat when I gun it but acceleration stops at about 1/3 throttle while rpms increase until the motor is running wide open. I suspect something is slipping although that is a rather uneducated guess. Any theories, test or help in general would be greatly appreciated.<br />Schluter