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PostPosted: Sun Aug 20, 2017 1:56 pm 
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Indicated between 800-1400 on taxing...unstable... seems fine on run up... showed 2700 on take off should be about 2600...appeared steady and stable at cruise... and indicated about 700 rpm after landing and taxing to dock. Which is it what it normally indicates...


Bad tach meter? Cable? Faulty governor?

Thanks



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PostPosted: Sun Aug 20, 2017 2:15 pm 
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Gauge, cable, tach drive. Could be anywhere in the system. Was this done within the last year?
Quote:
Standard 625 APPENDIX C - Out of Phase Tasks and Equipment Maintenance Requirements

8. Tachometers

The accuracy of mechanical drag cup type tachometers, for fixed wing propeller driven aircraft, shall be checked on site annually, and be accurate to within the tolerances established by the aircraft manufacturer or, where no tolerance has been specified by the aircraft manufacturer, to within +\- 4% of engine RPM at mid-point of the cruise range.

Is there any reason to suspect the governor? Does anything else fluctuate?



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PostPosted: Sun Aug 20, 2017 2:26 pm 
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Thanks

This a digital tach and yes it is checked every year. Never acted this way before...nothing else is fluctuating at all.



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PostPosted: Sun Aug 20, 2017 2:34 pm 
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Ah. Connected to a sender or hooked into the p-leads?


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PostPosted: Sun Aug 20, 2017 2:46 pm 
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I believe so but will have to check with engineer when he has time


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PostPosted: Sun Aug 20, 2017 2:50 pm 
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tazin river wrote:
Thanks

This a digital tach and yes it is checked every year. Never acted this way before...nothing else is fluctuating at all.

Could you actually hear the RPM fluctuations?


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PostPosted: Sun Aug 20, 2017 3:31 pm 
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No

Bad connection?



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PostPosted: Sun Aug 20, 2017 3:33 pm 
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Had pile of rain while the plane sat at the cabin? Moisture related?


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PostPosted: Sun Aug 20, 2017 3:36 pm 
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I spent ages (years, $$$$) chasing crazy intermittent fuel flow fluctuations. It turned out to be only instrumentation error caused by electromagnetic interference from a poorly shielded spark plug lead. I found it finally because it went away the instant I switched to run only on one mag.

Something to try in your circumstances.


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PostPosted: Sun Aug 20, 2017 6:21 pm 
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Sounds like an interference problems of some kind as photofly alluded to,

When its acting up try switching mags, power off the radio, turn off the alternator etc. see if you can find an offending source.

Did you have new electronics installed? Any new electrical work? That would be an obvious suspect.

A common problem is just poor grounds, that's often a good place to start. Sometimes the bolts where the common grounds attach gets rusty or loose.

A frayed connection is also possible, look for it at the point where the wire bends or where it is crimped, often the strands break one by one and pretty soon its making intermittent contact.



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