I have a Rotax914F with a standard Rotax 2 1/4" dial rev counter and a Fly Dat engine montoring unit.  I am finding that the revs on the Rotax     2 1/4" dial rev counter seem significantly lower than the revs shown on the Fly Dat which also shows fluctuations of up to +/- 150 rpm at one setting.   I have an MT CS prop fitted and am finding that unless I set the engine speed to significantly less than 5800, I risk overrevving the engine on take off unless I open the throttle VERY slowly.   The dial rev counter (which is all I need in theory) never shows engine revs overspeed or any detectable fluctuations in revs.

Does anyone else have similar experience or advice?

  • Re: Rotax rev counter

    by » 3 months ago

    Some time ago I did some accuracy checks on my Rotax tacho and it seemed to have various RPM bands that were accurate and others where it had an error. I'll see if I can find my notes that I made at the time. I seem to recall that it was sufficiently accurate at cruise RPM that I didn't worry about it.

    My friend has a CS prop on his 912 and he says that if he throttles up too quickly it is easy to over-rev the engine.

    If this is a new phenomenon then I would check the wiring, the connections and make sure that the tacho sensor gap on the flywheel is within spec (see the heavy maintenance manual). You could also make a resistive measurement of the winding as seen from the tacho end of the wiring and then tug on the wires, connectors and sensor just to make sure that you haven't got an intermittenet connection. Such a connection could give rise to additional pulses being seen by the Flydat resulting in the kind of readings that you are seeing.

  • Re: Rotax rev counter

    by » 3 months ago

    Thank you for that , it helps a lot.    I will cjeck the connections as suggested.  I am also planning to do some strobe checks to confrm the revs and will let you now what I find.   It is a bit concerning though that some may be relying on the dial tacho and regularly over revving without knowing it.



  • Re: Rotax rev counter

    by » 3 months ago



    You may be interested to know that I have now checked my dial Rotax tacho against a Trutach reference and find;

    Rotax Tacho

       Calibrated Tacho













    Which is a bit worrying – I only suspected a problem because I had a FlyDat comparison, which now has proved to be correct.

    So, I contacted Aviasport who make it and who kindly replied;

    In the back of the gauge, in the center , if you can find a round adhesive tape , remove it and with a small screwdriver you can adjust the RPM.   If not have the adhesive tape and have a pin, is only possible calibrate the gauge with software

    I then asked if this was normal (mine is about 15 yrs old) and they again kindly replied;

    Is normal.

    Keep in mind that to move the needle has several mechanical components, such as springs and coils. 

    These over time lose elasticity.



    In this light it might be wise to ensure that all Rotax owners are aware of this.


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