Tach

[gayala]

My tach started sticking a little so I took the cable out cleaned it, then I cleaned the cable houseing. Afterwards I put a light coat of grease on it I put it back. The problem pressisted and now it is stuck 2500 rpm. I had noticed that the cable houseing has a kink or break on it like in had been bent to far...My question is do you think it is in the instrument or in the cable housing? I was told it had to be in the instrument because it didnot return to-0-after the motor was turned off. I don' know if a stuck cable would hold the rpm at 2500 or not.

[GAHorn]

The instrument has failed. The mechanical tachs use a rotating magnetic clutch to spin the flywheel which is geared to the needle. When motion stops, the needle should return to zero.
When you replace the instrument, install a new cable as well. They are all available from Aircraft Spruce. 877/477-7823

[n2727d]

I lost the tach in our 170b about a year ago .I replaced the cable and replaced the tach with one from aircraft spruce .I had the hours rolled up to the right time for an additional fee. the new tach cable was not compatible with the new tach that I was sent so I had one made near here and it was not satisfactory .
To make a long story short I purchased an electronic tach and sent the other one back to aircraft spruce with a deduction for the hours rolled up on it .I have been very satisfied with the electronic tach . IMHO buy an electronic tach and install it and use it . I have had no problems with it.

[zero.one.victor]

I've always thought that a digital (electronic) tach would make it a lot easier to accurately lean the mixture. You could lean and watch the rpm go up,then when it started to drop richen it up slightly and aha-- there you'd be,just rich of peak!
I've heard that the digital tach readout will "flicker" (fluctuate the numbers) too much for this to work. I recently flew in a 170 with a digital tach,it seemed to read real steady but it had a Lycoming with c/s prop,maybe with a fixed pitch prop the digital tach wouldn't work as well.
Mark,any thoughts on this?

Eric

[GAHorn]

What was unsatisfactory/incompatible about the tach/cable, Mark? I've purchased tachs and cables from Spruce that were perfect replacements.

[n2727d]

George I purchased the new tach cable ,thinking that was my problem .Then when I had to have a new tach because it was not the cable ,the new tach and the old tach had different ends . So I needed another cable for the new tach . My old tach might not have been the original but we have not owned this plane but 2 years .
So I sent back what I could get some money back on and purchased the electronic one . The paper work went well ,with no problems .

Mark

[43a72v]

I don't have a digital tach in my airplane, but I believe the digitals take their RPM info from the mags, which shouldn't vary beyond what the engine is actually doing. A constant-speed prop equipped airplane would obviously be smoother than a fixed-pitch because the governor regulates the engine speed, but a fixed-pitch shouldn't fluctuate such that you couldn't lean out with the precision you'd like.

[GAHorn]

I've always thought that a digital (electronic) tach would make it a lot easier to accurately lean the mixture. You could lean and watch the rpm go up,then when it started to drop richen it up slightly and aha-- there you'd be,just rich of peak!
I've heard that the digital tach readout will "flicker" (fluctuate the numbers) too much for this to work. I recently flew in a 170 with a digital tach,it seemed to read real steady but it had a Lycoming with c/s prop,maybe with a fixed pitch prop the digital tach wouldn't work as well.
Mark,any thoughts on this?

Eric

Josh also left a message regarding this business of leaning with a Digital Tach and with regard to doing it with a constant speed prop.
With these two messages I thought I'd mention that leaning an engine with a constant speed prop cannot be properly done using RPM because the CS prop will govern at the selected RPM. The way to lean accurately with a CS prop is to use EGT.