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912ULS runs very rough above 4200 RPM

in 912 / 914 Technical Questions 10 years ago

  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    Hi Robert,
    normally a ignition issue is fairly consistent: a high/rough mag drop at 4000 will also be high/rough to a certain degree at all RPM's. To confirm: Run the engine on the bad mag and look for the cold cylinder (use EGT's or a laser temperature sensor or a simple temperature probe)
    Also the secondary coils are extremely reliable. They are very rarely part of the problem.

    Carb issues on the other hand change with RPM.
    Rob
      
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  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    Thank you for the help. I took the carbs off once again and completely disassembled, reassembled with new parts everywhere.

    The engine is running beautiful now. I guess I will never know exactly what the problem was, but it is gone now.

    Thanks once again - for "forcing me" to take the carbs apart once again. It worked :)
      
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  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    Hi Robert,

    I was with Rob on this and was leaning hard on a carb issue especially since that was the last maint. and then the problem started.

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

      
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  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    Hi Robert Could you advise what parts you replaced. I have been experiencing the same with my 912 UL. It had not run since last August due to work commitmets. Although I had been giving it some starts and runups, but not to max rpm.

    Then 4 weeks ago I went to prep for a fly and it started missing above 4,500 rpm, pulled the choke enricker and pulled good high rpms.

    I am awaiting the 'o'ring kit then will clean next week.

    I have looked inside and its clean, fuel was new, and fuel flow to carbys right, all jets etc clean.

    Yesterday I altered the float bracket a bit and the mis develops eariler now at 4,100 rpm. Weather is raining here at present so may be tomorrow when its finer I'll adjust the float brackets and expoy the top cap outside area of the carby.

    I'm also going to buy the float level guage tool from Rotax so I'll alwys have that exact. (Should have bought it after the last service.)

    My jet needle, needle jet and carb rubbers all look excellant. engine has only 430 hours and 1994 version.

    Thanks in advance for any information.

    Safe enjoyable flying

    Mike
      
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  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    Mike,

    I replaced all the parts that come in the rebuild kit; slide diaphrams, gaskets, o rings for the needles, main jet needle, float needle valve ....

    My opinion is that you need to pay CLOSE attention to all the things the other contributors talked about like slide/diaphragm alignment issues and location of venturi reference holes, all passageways being clear, right shafts in right starting carb, butterfly assembly etc, etc.

    I honestly don't know what I did differently on my last assembly job but it cleared up the problem.

    Wish I could help more.
    Thank you said by: Michael Saunders
      
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