At full throttle motor sounds like it hits a rev limiter. I have had and worked on 70's models and never ran into this issue. It has a cd2 box on it, is that why it' doing that???
And has good compression has good spark has no problem starting runs and idles great takes off like a rocket and planes out quick. Motor runs good just at wide open throttle it sounds like it hits a rev limiter you can back the throttle off about a quarter turn and runs perfect but as soon as you Hammer back down on the throttle it sounds like it's hitting the rev limiter.
What's on a 1442 Jon Boat I just change the prop out today to a 13 pitch. Might have figured out the issue the adjustment screw for the idle on the side of the motor by tiller handle was long and it was hitting the side of the motor not letting the Armature plate hit the wide open throttle stop. Going to put it on the water and try it