The Cessna 120-140 Association Forum

My tachometer started acting up last fall. RPM OK but hours were not recording accurately. I would fly .8 and get .2 or .3 on the hour meter.
1. Ordered new Mitchell 2400RPM average cruise tach. Could only get 2300 RPM anytime. Usually got 2500 or so on takeoff and had to reduce power at level off to avoid going beyond 2575 as I have a climb prop. Changed tach cable and new tach housing oil seal.
2. Sent Mitchell back, bench checked OK. Reinstalled - same result only 2300. Checked both tachs with a One touch strobe. Mitchell tach low, old tach right on.
3. Mitchell sent me a new tach, same model, same result low RPM today. I think I'm giving up on Mitchell. Any thoughts on what might be the problem? Can the old tach can be repaired? Photo attached, manufacturer unknown, no markings.

Frustrated, but getting very proficient at tach removal and installation.

Since the Old tach was faulty recording time, I would suspect the accuracy of the rpm.

You should have the Old one tested before giving up on the Mitchell.

Rick

If Mitchell tested the tach and said it's OK, I'd look very seriously elsewhere. Did they give you a calibration chart? Can you give more specifics about the strobe test?

I've had the airplane, C-85 12F with a climb prop for 16 years so I kind of "know" what 2500+ sounds like on takeoff and what 2400 sounds like in cruise. When I installed the new tach I knew it was indicating slowly based on sounds, throttle position on takeoff and in cruise, etc. I checked both tachs with a One Touch optical hand held device in flight. The One Touch is a hand held RPM checker that can check 2, 3, or 4 blade prop rpm, you just point it at the prop and it gives a digital read-out. I didn't do it on takeoff because I was flying solo.

In what I knew to be "cruise" based on experience, the Mitchell was indicating 2200, but the hand held showed the prop was going at 2400. I put the old tach back in and it was indicating correctly at all RPMs according to the hand held device. I did get a calibration card with both of the tachs that Mitchell sent me. Mitchell was very cooperative, they replaced the first tach that I sent back for the second time with a new one that performed just like the first one...indicating slower than the prop.

Thanks for the input guys...this is frustrating.

Interesting that there would be two treads at the same time concerning tachs that read 2200 RPM... viewtopic.php?f=7&t=602

Yes, you can get yours overhauled. I'm not sure who can do it; I'd try Rudy Instruments first. They've always done good work for me and others here. If all else fails, a shop that overhauls classic car speedometers should be able to handle it; after all, that's exactly what it is...

Thanks John. Will try Rudy's. I've had the tach apart. I fastened it in a vice and chucked my drill to a cable and watched the "workings" turn at about 2200 which is as fast as my drill would go.
There's a brass gear and a plastic gear on the right hand outer drum that don't quite mesh all the way around and I think that's the issue. I didn't dare take it apart cause I at least can fly the plane the way it is, as it does give me good RPM indications. I have a hobbs to record my flight time and I can estimate the engine time.

We used to use tach times 1.3 = hobbs, so hobbs/1.3 ~= tach. (Or, hobbs times 0.77)

The tach drive runs at 1/2 crankshaft speed, so 2200 RPM on the drill should indicate 4400 RPM on the tach...

Sorry - I meant that my drill would only go fast enough to show 2200 on the tach. Thanks again for the info.

Rudys first - I'll be surprised if they can't. If not, try Keystone Instruments, you'll find them on the net - located in PA Piper country and they do about every thing including the old original Cub stuff. Excellent reputation.

Took the old tach to a local classic car speedometer repair shop. Bob's in Howell MI. They replaced one of the damaged drums and now I will go check it out.
I returned the Mitchell tach. I am still not sure why the Mitchells did not work, but it was clear that they indeed tried very hard to solve the problem.