oooFlew last night to reestablish my night pax carrying currency. Failing tachs must be going around. Although different failure mode for me. The RPM needle on my tach was fine and was stable on my selected RPM. But the recording (hours) part of my tach didn't advance at all. I'm guessing just an internal tach issue. If so, I'll order another set to the correct hours.
That much flicker implies a seizing tach. Undo the tach cable nut on the back of that tach. See the short shaft sticking out. See if it spins freely. As Magman says, older tachs had a tiny oil hole just above that shaft. You can wick some really light oil (like sewing machine oil) into it and see if it settles down.This is the cable binding, not the tach failing correct?
And the OP should do himself another favor. Don't buy another mechanical tach. There are good electronic tachs available that don't need that cable, they don't jump around, there are no moving parts to wear out, and they never go out of calibration. Here in Canada we have a requirement for an annual accuracy check of all magnetic-drag-type tachs, which is pretty much all of them, and the deviation limit is 4% of the reading in the middle of the cruise range. The magnet that drags the cup and needle around gets weaker with age, and the tach begins to underread. 4% of 2500 is 100 RPM. Replacing those old tachs gets around that annual check, and avoids the aging-magnet and worn-bearings and seizing shaft and chattery cable and leaking tach drive oil seal stuff.Just in case you you have to replace the Tach, please do everyone a big favor.