I only know about the tacho on the K12's but there may be some similarities.
Is the tacho electronic with wires, or mechanical with a cable?
If the tacho is electronic, it may have a deliberate physical offset below the zero mark which is set correct when the unit is powered.
In both cases the tacho is a delicate 'moving coil' meter built like a large clock. If you are sure that the tacho does normally read at zero with the key off then this is what you have to do:
First check you have not bent the needle and it is touching the front glass. Take it out and make sure the pointer swings freely - use a wooden toothpick to move it.
Then you need to remember if it does sit at zero with the key off, or is biased past zero.
Locate the delicate mechanism attached to the pointer. You will see two hair springs top and bottom like in a mechanical clock. First check you didn't get a coil or two overlapped, you can carefully separate them with the toothpick but don't put in a kink. Also check with a magnifying glass that both ends of the hairsprings are actually attached.
One hairspring sends the pointer anti-clockwise and the opposite identical spring sends the pointer clockwise. Working normally, the springs are set for balance which may be zero or an offset bias which is corrected by the electronics.
On one of the springs on its outer fixing point there is usually a 'tab' which can be rotated. This sets the tension on that spring so the pointer can be zeroed. It may be marked or sealed with a dab of paint.
When you have checked or made corrections you should really check the tacho readings are accurate at different rpms, it's easier with an electronic unit than cable driven. But if you are a zero or redline monkey it probably doesn't matter!
Here endeth your lesson on fixing clocks!
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