My Honda NTV 650 Revere, after 25 years of duty, has a very loose side mirror: i need to adjust it every time because vibrations and wind speed move it.
I searched the net for a solution and I found several misleading recipes: using Loctite or even drilling an hole through the ball joint and blocking it with a pin! The solution is very simple indeed if you know how the mirror is assembled and how the ball joint works, so here you are two photos:
Without removing the mirror you can try to rotate it clockwise, so that the nut is dragged by the spring friction to tighten itself. But if the self-locking nut has lost its grip, the solution cannot last for a long time.
So you can remove the mirror. Be carefull to pull the rubber frame very gently to avoid breaking the mirror. Separate the rubber from the metal body a little at a time, using two or three screwdrivers and acting simultaneously on several points along the short side of the mirror.
You can then use a new self-locking nut, may be reinforced with Loctite or with a lock nut. I also added a washer to reduce friction with the spring.