"Rich S." wrote in message
...
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.



An old gent I know who has been flying longer than anybody offered the
observation that the reason that old airplanes don't fly as fast as new ones
is that the needle return springs on tachometers and manifold pressure
gauges get weak with age causing the instrument to over-indicate. If pilots
keep setting power at the same gauge indications the airplane will indeed
fly slower as it ages. Re-calibrating the gauges restores the youthful
vigor.

If only that worked for me.

Bill Daniels