During cruise the RPm suddenly drops appr 150 rpm and goes up again after 5-10 seconds. It happens like once a minute.

Has anyone got the same problem or any Idea of where to start looking ?

My own thoughts: As it resembles a drop at Mag Check I speculate in ignition problem. A loss of L or R ignition system would give a larger drop. But i guess it could be a part of L or R that is failing ? A test I plan to to is to shut off L and R when it happens as see if the engine stops due to that (seaplane over a lake). If so I know that half of the Ignition system was out before the test. 

There are several other potential problems of course for example Carburator. A sudden change in mixture ? But wouldnt it run rough then ?

Rotax 912 ULS  500 h






  • Re: Sudden RPM drop at cruise

    by » 2 years ago

    Food for thought:

    A 150 rpm drop is not  large enough for one whole ignition system failure. This could be a plug or plug wire? It could be fuel, but the rpm drop doesn't seem big enough for that, but you never know. Is the fuel pressure fairly steady when this happens? Rule out the plugs first and trim back the plug wires about 3/8", apply a dab of dielectric grease and screw the caps ALL the way back on. Change the plugs. Then give it another test run. Do the easy, common and simple test first. Could it be carbs????? Mmmmm possibly, but not number one on the list. You could always throw a set of gauges on them and take a look if the plug test doesn't pan out. Use gauges because electronic sync devices can't diagnose as well as gauges. 

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

  • Re: Sudden RPM drop at cruise

    by » 4 months ago

    I know this was a couple of years ago but did you find out the cause?

    I'm having the exact same issue on a Rotax 912 80hp with ~800 hours

  • Re: Sudden RPM drop at cruise

    by » one month ago


    I had the same problem of suddenly drop of 150 to 200 RPM`s in flight and the cause was a LOOSE CONNECTOR from one of the ignition modules (causing a false contact) checking carefully i realized that it was out of the steel fastener, so i held it on and put aditional zip ties to keep all the cables and connector fixed.  

    Hope it helps you.

    Angel Galicia

    Thank you said by: RotaxOwner Admin

  • Re: Sudden RPM drop at cruise

    by » one month ago


    Thank you Angel for sharing your experiance about this. What I did was to change spark plugs and tightened the screws in the spark plug caps. After that I did not have the problem om sudden rpm drop. But of course I could have touched something else, for example a bad connector and that could have had an impact. Hope you solve it also Stefano. 


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