Not really sure what you are trying to say with this metal cover thing...but the tach will act strange if the cable isn't lubricated. When the lubrication gets to the point where it starts to stick, the cable will start to get stuck for a moment and this will translate into a Tach that has a needle that jumps all over. Eventually, you will be buying a new cable if you don't disconnect the cable from the bottom of the Tach and pull the flexibel cable up and out of the tube and lube it with an oil and grapite mixture, just don't lube the upper inch or so of it when you replace it in the tube, as the oil will creep up the cable and fowl the inside workings of the Tach. Make sure it is seated in the little end piece at the bottom on the cable tube when you replace it, it should not just turn loose...got to be seated correctly. You can use plain oil, but sometime soon, get a bit of graphite powder, mix it with light oil and redo it. Then it will be good for about a year or so.