A friend of mine bought a Harley Speedo/Tach combo for his 07 Softail Heritage. The tach seems to have went out to lunch after a couple of years now. I guess it is a far gone conclusion that the tack is not fixabel?
You might get lucky and it's just a bit of oxidation or bad contact on the plug. Tach is driven electronically, so a pinched wire or bad contact can cause problems.