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912UL Running Rough

in 912 / 914 Technical Questions 11 years ago
I'm about at the end of my string.

I have a mid 90's vintage 912 UL, 80 hp. 70" warp drive, taper tip prop.

My 912UL is running rough from 2000-3000 rpm. (Kitfox IV)

What I have done so far:

Synchronized Carbs
Dynovibe balanced prop
Removed, cleaned and synced carbs
Removed, cleaned, rebuilt and synced carbs
Rebuilt engine mounts
Disassembled and reshimmed gearbox
Replaced gearset and dog ring reshimmed gearbox
Re-synced carbs.

The vibration has gotten better, but it's still not what it was a year ago - smooth at all rpms.

I get vibration when throttling back on downwind, or whenever the rpm is in the 2000-3000 range.

The motor will idle at 1800 smooth as silk. Anything above 3000 rpm is also smooth. The engine makes good power and the plane climbs like crazy.



I'll listen to any ideas at this point… :unsure:

  • Re: 912UL Running Rough

    by » 11 years ago

    Food for thought,

    Kitfox had problems with their engine mounts and have a modification. Have you done that? It is more or less shortening the spacers between the rubber mounts to allow for more compression on the rubber mounts themselves.
    You may also just have harmonics at that certain rpm with that prop under a specific load and there won't necessarily be a fix. What is your wide open throttle rpm in flat and level flight? You may need to just unload the prop, plus the Warp drive is a heavy prop it it may be prop caused. You really should stay at 68" or under with a Warp and never nickle edges. You would be better off over all with a light composite prop that has been dynamically balanced. Most light composites now days don't really need a static balance. If your Warp is a 2 blade prop a three blade may also improve vibration issues.

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

      
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  • Re: 912UL Running Rough

    by » 11 years ago

    Yes, I did do the mount modification per the Kitfox SB.

    My error, the prop is a 68", taper tip, 3-blade. At wot, I can over rev the engine in level flight. Takeoff I'm showing 500-5100 at initial application of full power.

    The frustrating thing is I have 600 hours in this plane and this issue only showed up last summer. Over the last 50 hours of run time I have struggled to overcome this issue.

    Is there a lifespan on plug wires or boots that I need to look at? Plugs are new (12 hours).

    Something else I'm missing?
      
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  • Re: 912UL Running Rough

    by » 11 years ago

    Try a mag check when it is running in the rough mode: does it change form one mag to the other? if not then a ignition problem is unlikely.
    It is probably the carbs: Do you have a friend with a 912UL or service center nearby that you could borrow a pair of carbs to troubleshoot?
    How is the slide? any chatter marks/metal transfer between carb body and the slide? If so, this is an indication of prior rough running and it can cause a seperate vibration in the 2000-3000 range.
      
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  • Re: 912UL Running Rough

    by » 11 years ago

    I've gone as far as replicating the rough running in flight and did a mag check. I don't think it's ignition, but I've been through the carbs so many times, i don't know where else to look.

    I have had the thought to fly the plane down to LEAF (I'm in northern Wisconsin) and have them do the carbs. The only draw back is the large $$$ that will ensue.

    But at this point I really don't know what else to do...
      
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  • Re: 912UL Running Rough

    by » 11 years ago

    If the carbs are badly worn at the slide/body contact surface replace the carbs. We have tried dressing out the surfaces, teflon coating and just replacing the slide but unfortunately it does not cure the vibration problem.
      
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