Tachometer gives false indication in flight

[M20 Ogler]

Working fine on the ground. But after takeoff it continues to read 2700 rpm even after setting cruise climb power. The governor is working because I can hear the engine speed responding normally when I move the prop speed knob. This is the old style mechanical tach on my m20c 

Is there some kind of pixie dust I can sprinkle on it to make it act normal? 

[kortopates]

Time for an EI R1, very affordable too:

https://www.aircraftspruce.com/catalog/inpages/ei-digitalgraphic1.php?clickkey=5825967


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[0TreeLemur]

30 minutes ago, kortopates said:

Time for an EI R1, very affordable too:

I have one of these that my A&P removed from our J model in April.   PM me if interested.  I don't need it. 

[Vance Harral]

The EI R1 is a reasonable replacement for a mechanical tach: accurate, inexpensive, uses no mechanical cables.

It is also, in my opinion, a pain in the a** to actually use.  One of the flight school airplanes I teach in has this tach.  It's difficult to read from the right seat in daylight, due to a combination of the way the backlight works (might be an installation error), and the LCD viewing angle.  The ring of LEDs around the perimeter of the instrument are supposed to provide an analog-like "feel", but they aren't marked with actual numbers, and there are only 16 of them to cover a range in excess of 1600 RPM, so they feel useless to me - I never look at anything except the numeric display, which I feel like could have been better without the lame ring of LEDs around the perimeter of the face.

Finally, you of course can't read the tach time on the instrument without powering it up, and the interface to read it requires cycling through two "pages" of display once you have more than 999 hours of tach time.  This is probably less of an issue in an owner-flown aircraft, but I'd guess 90% of pilots at the flight school forget to make a note of tach time before turning off the master, and therefore must cycle the master again later to get tach time.  This puts almost twice the wear on the master switch, and is a recipe for accidentally leaving the master on.

If you're looking for an inexpensive tach that doesn't require a mechanical cable, I think the UMA TSO'd unit is a better choice: https://www.aircraftspruce.com/catalog/inpages/tso_tachometer.php