The needle suddenly started to stop at 2000rpm and the after a second or two, jump up to the correct higher rpm but only at load (driving).
At the same time I did rebuild my ignition to "Wasted spark" using a coil package instead of the single ignition coil which made me suspicious if the tachometer couldn't handle the signal.
I still fed the coil from 12V+ trough the tacho in the same way as before the rebuild which should work in theory at least.
Since I couldn't find any explanation to the stickiness I bought a "RVI to RVC conversion board" from Spiyda which uses the 12 square wave from the EFI system but the problem was still there.
today I dismantle the tacho again and opened it once more and took a close look under a magnifying-glass and found that the spindle which the needle is attached to is bend. I did manage to bend it back to make it a little straighter and also screwed the brass bearing a little counter wise to make some more play/space for the needle to move and voila... it works.
I don't know if the picture is clear enough but the spindle is bend and also the brass bearing is shown.
