If you determine its an electronic problem not mechanical and the needle is fluctuating, as opposed to vibrating up and down with shake. Its a sign that the alternator is starting to go out.
Unusual, but its happened to me on another bike.
If alternator needle goes to zero and bike is running-it means alternator has failed and battery is not charging. Unless you disconnect the headlight, you'll run the battery down and be stranded very quickly.
If you keep the revs up (by sound-I know you won't be able to see it on the tach) it is possible to keep the bike going, but once it dies-you are done.
The way to confirm this, is rev the bike from idle, if your headlight brightens even a tad you are good. If headlight stays the same or starts to look dim, its running directly from the battery. If you are directly on the battery avoid using any volt, don't indicate, don't stay on the brakes (LED brake lights don't take much, but every volt helps get you home.)
Very infrequent problem, caused by the alternator brushes getting dirty, happens if you run in dusty areas or lots of dirt roads.
As others have pointed out there is no speedo or tach cable and it would have to be vibrating really badly to upset the tach needle-if its vibrating that bad you'll risk damaging the engine.
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