1990 Force 120HP carbs mix adjustment help

[Nordin]

Hard to say when not have the engine to look at.
But I think you problem are in the LU.
The pinion gear have to have right play/back lash.
Too tight the pinion gear will press at the forward and reverse gear.
But as it idles in neutral at 800 RPM and the press are at the F and R gear even in neutral, I would guess you have a too tight end play in the prop shaft.
The nut holding the bearing cage assembly for the prop shaft should be tighten so it will bottom the cage.
I interprets it as you have to loosen it a bit to be able to turn the prop shaft.

 

[Nordin]

Have not looked at the wideo cause I can not and will not login to drop box.
Pulling the plugs and put the drives in forward gear and then compare how hard it is to turn the flywheels may give you some clue.
But you are turning the impeller too and depending how flexible the impellers are it might give you some wrong feeling about how hard to turn.