You may need to clean and re oil the cable and the input shaft of the
Tach.
If it chatters then the tach will read high for a while, then it will
read ZERO when the cable breaks. You can feel this vibration if you
touch the front of the Tach when it happens. If this does not fix it
you may have too much clearance in the Tach bearings.
john

On Wed, 3 Nov 2004 13:23:05 -0800, "Rich S."
wrote:

So, I was flying back home, happily burping all the shellfish I ate at the
Shelton Oysterfest, when my engine started speeding up. Well, it didn't
really - but my tach said it did. Without touching the throttle, I went from
2400 rpm up to 3400. Funny, my airspeed didn't change.

It looks like my sturdy, simple mechanical tach (bought used at OSH) has
gone South on me. I pulled it out and removed the case. Everything inside
looks great. It is cable driven through a magnetic coupling just like a car
speedo. The needle has a return spring wound in a coil like a clock spring.
I assume the calibration is done by setting the needle on its shaft in a
position relative to the return spring, so a given rpm matches the needle
position on the dial.

I thought I would discover a broken return spring, but it looks okay.
Turning the drive with an 1100 rpm electric drill shows ~2500 on the dial.
If I allow the needle to bypass the stop pin and add an extra full turn of
tension on the return spring, the tach shows the drill turning ~400. It
looks like the needle has slipped on the shaft releasing about a half-turn
of spring tension. Either that or

A. the spring has developed a weak spot or
B. the other end of the spring has come unsoldered from the frame.

I don't think it's "B" because the entire spring would be unwound. Visual
examination of the spring doesn't show any defects.

Any ideas?

Rich "May as well take it apart - it don't work" S.