I have a 1963 40hp Johnson seahorse motor and I really appreciated all of your helps. I couldnít get the engine to goes above 6 miles/hrs in full throttle. When itís in full throttle engine reps up and look likes it started to speed up but less then 10 secs rpm dropped boat flatten out, this repeated constantly at full throttle. However, if throttle to lower speed the reping stop. I have replaced water pump, gas lines, new sparks plugs, gas filter, new throttle cable, rebuilt carbureted (all valves needle are still in good condition) and reset it back to original factory setting. Don't know what to do now.