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Hello All, I have an issue with my Rotax 912ULS engine. Total 60 hours. The prop is an Airmaster c/s.
When in cruise around 5000 rpm the engine suddenly runs a little rough for a second or so. I can feel small ‘kicks’ from the engine and the rpm drops 50-100 rpm. But just very short before it goes back to normal, but just to misfire again after a few seconds or sometimes after half a minute. I don’t feel anything when under full load at T.O. I do not have a drip-tray (heat shield) under the carburetors.
Have some of you an idea about in which direction to trouble-shoot.
  • Re: Rough running around 5000 rpm

    by » 7 months ago


    Let's go back to basics when diagnosing the issue. When you're operating at that RPM, you're essentially drawing all your fuel through the main jet. We recommend performing the following checks:

    • Inspect the float bowls to ensure there is no contamination inside.
    • Verify that the main jet is clear and unobstructed.
    • Examine the floats for any signs of damage or irregular weight.
    • Check the condition of your filters and inspect the water trap (gascolator).
    • What are the results of your mag drop tests, and have you examined your spark plugs?
    • Assess the resistance of your spark plug caps and ignition leads.
    • Inspect the pin connectors of your ignition module for any loose connections.
    • Carburetor balance
    • Propeller balance

    If you upload a photo of your engine, we may be able to identify an issue (or potential issue) 

    If you don't feel comfortable or lack the appropriate tools to perform these tasks, it is recommended that you take your aircraft to a trained Rotax mechanic (iRMT) to assist with diagnosing and resolving the issue. To locate an iRMT in your vicinity, you can click on these two links: 

    https://dealerlocator.flyrotax.com/

    https://www.rotax-owner.com/en/support-topmenu/find-service/rotax-irmt-service

     

     


  • Re: Rough running around 5000 rpm

    by » 7 months ago


    You don't state airframe, age, etc. which may play a role. I had similar symptoms, getting progressively worse over 2 years, due to an air leak through the stem of the fuel tank selector valve. Some of my other posts discuss this in greater detail.

     

     


  • Re: Rough running around 5000 rpm

    by » 7 months ago


    Hello Jeffry

    Can you tell us if you are getting the RPM by throttle setting or load of the propeller with your Airmaster?  The major item that changes over time is your gearbox will wear in and reduce the tension internally.  If this is too low at peak torque areas, 5000 to 5200, it can start to move in the overload clutch with bounce from the reduced tension preload.  

    One simple check would be when the engine is warm on the ground at idle, 1400 to 1600 RPM for example, is it making a rattling noise?  If it is then fix the shimming of the gearbox and try it again.  It is not uncommon to see issues, especially on reducing power from high RPM to cruise, to get a vibration at the peak torque settings on a worn gearbox. 

    Given that the engine is in mid range of the carburetor settings perhaps see that the jet needle is not vibrating in the needle jet.  Check the circlip position as well.  The factory setting is number 3 groove but in hot weather number 2 might be better.   it is always good to sync the carbs and ensure that the midrange is running balanced.  (mid to full open is the most important) 

    Let us know what you find.

    Cheers


  • Re: Rough running around 5000 rpm

    by » 7 months ago


    The status of today:

    I have checked the bowl- no contamination
    Have not checked the main jet yet - during take off and climb with the highest fuel flow I have no misfire issues.
    Checked the floats: 3.0 to 3.1 grams each and no damages.
    Filter/gascolator are absolutely clean.
    Drop test: drops 75 rpm for both positions.
    Have not checked the resistance and pin connectors.
    Carbs are balanced.
    Propeller not balanced.
    Purchased 2 new ignition units, but not installed them yet (received them last Friday).
    Checked the situation yesterday: I did make a climb with about 90 kts. The temp held on 90 degree C and there were no issue after levelling out.
    Seems to be more prone to make the small 'ticks' when the engine temp is above 110 degree, so now I hope that installing new ignition units and install them away from the motor may help :-)


  • Re: Rough running around 5000 rpm

    by » 7 months ago


    Hi John

    Consider this.  The engine runs on 2 ignitions.  Just change one and try it.  Even if one completely fails it should run fine.  The problem you describe would require 2 plugs in the same cylinder to stop intermittently over and over.  Each head has one plug on A and one plug on B side.  This type of failure is highly unlikely in my opinion.  I don't know if you have carb heat but if you do try turning it on when this happens.  (it will enrich the mixture) it may help you diagnose the issue. 

    Cheers


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