Mechanical Tach Time Quit

[SuperDave]

My mechanical tach just recently quit advancing the tach time and now the rpm's are starting to vary. The engine was overhauled 20 hours ago and at that time I lubed the tach cable. I recently saw oil in the tach which didn't surprise me due to the lube job but I'm wondering if the oil has caused the problem since the tach only has 170 hours on it. Does anybody know the inner workings of the tach and whether the lube caused caused the failure?
Also, I recently have been flying inverted which I'm sure is contributing to dispersing the lube throughout the tach.
There doesn't appear to be oil coming through the tach drive on the back of the engine either.

 

[Far North Texan]

Mine did that. I sent it away to be rebuilt. The oil was coming thru the tach cable. After repair the RPMs work fine but the time moved backward for a few hours and then stopped all together. Interestingly the RPMs are dead on as I have a Proptach and can check it.
I bought a Flight Data Systems electrical tach that I will put on when I can work without gloves and a snowsuit.

 

[olyolson]

Tach

My buddy has a -4 that had mechanical tach problems also. He sent it to Mitchell Aircraft Products for a rebuild and bought a new tach cable. Similar problems and a worn out tach cable end. Works perfectly now.

 

[SuperDave]

Thanks guys!

I had the same thing happen yesterday after flying .5 hours, the tach time had actually reversed .1 but only time will tell if that trend continues.
I checked the Mitchell site yesterday to see if they offered repair but I didn't see anything. I may just go electronic and eliminate the whole issue.
You wouldn't think that getting lubricant in the gauge would be a problem but that's what I get for trying to do a good job.

 

[Vac]

Give Mitchell a call. Great folks! They charge a flat bench rate to fix the hour meter.

Cheers,

Vac

 

[SuperDave]

I called Mitchell today (it's one of theirs) and they said rebuild is $140.
I took the unit apart and the problem is there is a gear pressed onto a shaft that slid out of alignment with another gear that drives the hourmeter. I was able to slide the gear back into position on the shaft but I am sure it will slip out of alignment in a short period of time.
If push comes to shove I'll have to send it to them to do whatever it takes to keep that gear in place.

 

[blastinbill]

I had two Mitchell tachometers that did that when still new 17 years ago and I successfully applied a superglue-type product to the shaft where the gear is pressed-on to hold it. No problems on either one since.

 

[zilik]

My Mitchell tach started showing fast, like 3000rpm on the takeoff roll. I strobed it against a friends prop while in flight to confirm it was the tach and it was. It registered zero on the return to the airport. I figured it suffered the same problem that the speedo in my 67 mustang had. The magnet was dragging on the cup. I took it apart and the magnet had worked loose from the shaft. I little locktite and I pressed he magnet back on the shaft. Checked it with a prop balancer and it was dead on.