I have a 1984 evinrude 70hp on a 1989 truline sportcat fiberglass vee hull, the prop is an aluminum 12 3/4 x 15, it will only run at 28mph per gps at 5000-5200 rpm, ran half a can of seafoam thru it, champion plugs, the motor is mounted on the third hole down on the transom, I thought maybe the...