This fixed it for me:
https://www.forumguzzi.fr/viewtopic.php?f=8&t=7643These were the steps I took, and I was careful the whole time as all the parts need to be reused:
1. Pull the two allens that hold the dash onto the headstock.
2. Pull the four allen head screws that hold the plastic back on the tach, speedo, and center section. There are two on the speedo side, and two on the tach side.
3. Pull the three phillips head screws that hold the metal bracket to the individual tach unit.
4. Unplug the electronic connector from the tach. There is an obvious area near the bottom of the plug that needs to be depressed in order to disconnect the plug.
5. Pull the rubber o-ring that touches the chrome tach ring away from the chrome.
6. Carefully pry up the rear edges of the chrome ring from the side of the tach with a small screwdriver. The front of this chrome ring pushes a big rubber ring against the tach's glass, which is pushed against the plastic tach housing. Once the ring is opened up enough, pry the ring forward until it is off of the tach. Be careful here.
7. Remove the rubber ring and glass (clear plastic really).
8. Manually push the tach needle so that it is at the bottom-most line, one below the 0.
9. Plug the tach into the electronic connector and make sure the needle goes to 0 when you turn the bike on. If you adjusted too far in the prior step, push the needle forward until you feel some resistance. That resistance is the shaft for the gauge maxing out, and the needle rotating on it.
10. Press the round button where the needle connects to the shaft down. This will anchor the needle to the shaft better.
11. Turn the bike on again and make sure everything is where you want it.
12. Disconnect the tach from the bike.
13. Put the glass and the rubber back on the tach and fully seat them.
14. This step is a pain, but get the chrome ring back over the plastic lip on the tach. The rubber being fully seated in the prior step will help. I found it helpful to try to push down each part of the chrome ring at the same time. If one section of the chrome ring is fully on, getting the other side of the ring over the plastic lip is very difficult. On the other hand, if one section of the chrome ring is just barely on, getting the other side on is much easier. Take your time here.
15. Bend the back-side, of the chrome ring, meaning the part that extends towards the rear of the tach, back on. Basically, undo what you did in step 6. Be slow and careful.
16. Perform steps 5-1 in reverse.