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912UL Rough Run Engine

in 912 / 914 Technical Questions 10 years ago
Hello Rotax owners, I need your help.

I have 80hp motor fitted in Eurostar plane. Motor has always been smooth. 20 hours ago motor starts being rough at 3000rpm. Each flight, rough run is more than last flight. Rough run becomes at 4600rpm when cruising.

I am not liking rough motor so I stop and test components:

  1. Prop - static balance, blades angles - OK.
  2. Gearbox - remove, new springs fitted - OK.
  3. Carbs - remove, fit new rubbers, jets, needles - OK.
  4. Crank - measure phase of pistons - OK.
  5. Throttle wires - OK.
  6. Sparking plugs - fit new - OK.
  7. Plug wire caps - OK.
  8. Compression - leak test - OK.
  9. Motor support rubbers - OK.


I have no more ideas for tests to make.

Every flight I make, rough motor is at different speed, 2000rpm and next flight maybe 2800rpm. Motor is rough on both ignition circuits.

Motor is 2003 with 800 hours.

I want my smooth motor back :-(

Any ideas everyone?

Thank you.

  • Re: 912UL Rough Run Engine

    by » 10 years ago

    I forgot also to say, there is no power loss when motor is rough, maximum static motor speed is correct, take off and climb is good like when motor was smooth.

    Sometime idling motor is smooth sometimes rough.

    Maybe this meant that problem is not serious?
      
    Report

  • Re: 912UL Rough Run Engine

    by » 10 years ago

    Today I make more tests. Tests were:

    1. Run motor with no exhaust muffler - no change to problem, motor is rough.
    2. Run motor with no prop - no change to problem, motor is rough.


    Should I replace motor with other used motor?
      
    Report

  • Re: 912UL Rough Run Engine

    by » 10 years ago

    Have you synced the carbs at idle and around 3500 rpm? If not do this first.
    Have you checked for debris in the carb bowls?
    Have you checked the prop blades for equal pitch, tracking and a dynamic balance?
    If the rubber motor mounts are 5+ years old then yes they should be replaced. As these get hard they loose their dampening affect. I always replace these at the 5 year rubber hose change interval.

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

      
    Report

  • Re: 912UL Rough Run Engine

    by » 10 years ago

    Thank you Roger for suggestions.

    Yes, sync at idle and 3500rpm - both OK.
    Float bowls tested - no debris.
    Prop blade angles (3 blades) < 0.3 degree error.
    Blade tracking < 0.5mm.
    Static balance - OK.
    No dynamic balance - no person with equipment close.
    Motor rubbers are old but not hard.
      
    Report

  • Re: 912UL Rough Run Engine

    by » 10 years ago

    For purpose of update of my progress: I have now tested motor with 2 different carbs from a different 912UL motor. After balance of carbs motor is still run rough so problem is not carbs.

    Tests have shown fault is not these items:
    1. Carbs.
    2. Prop.
    3. Gearbox.
    4. Ignition timing.
    5. Sparking Plugs.
    6. HV cables.
    7. Exhaust.
    8. Valves/piston rings.
    9. Rubber mounts.


    Next actions are open up motor to inspect inside. I will post my observations to help other owner who has same fault.

    Grzeg
      
    Report
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