Bill makes an excellent point here. I have also had cam followers that were bent slightly and would not allow the throttle plate to fully open, even with the cam adjusted out. Where you have had multiple powerheads swapped around, you may have run into this as well. Worth looking into.
You are learning a lot about these 22 cube motors. I had a 1959 Fastwin that was the best running motor I ever had. The fiberglass cowl rattled a bit at slow speed, but that motor ran the best of many. The separate Hi and Low speed adjustment is miles ahead of a fixed jet motor.
The actual roller is a lot less worn than the one I had on the old powerhead. There is a little bit of wear where these two components lock together.
I’ll double check the "link and sync" positions.
About the rattle. I’ve been fighting a few too. The loudest one I discovered was actually coming from the boat. The motor was sending harmonics through the hull. I had moved the motor up a little higher on the transom so that the clamp wasn’t resting on the top of the transom and the sound went away. I plan to put some "stall mat" rubber on the top of the transom to isolate the noise. I’m also thinking about putting herculiner on the underside of the cowl, and maybe some other kind of sound deadening. Does anyone know how the air is supposed to enter the cowl for the intake to use? I know there are small drain holes in the pan.. not sure if that’s all the motor needs?