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912UL Engine vibration on throttle back

in 912 / 914 Technical Questions 12 years ago
Gents I need some advice.

I have a 2003 Evektor Eurostar with a 912UL running a Woodcomp Klassic 3 blade fixed pitch prop. The engine has done about 700 hrs. Max static RPM is about 4800 and idle about 1400.

The engine has been well maintained and has worked flawlessly since I bought the plane 4 years ago. Just recently I have noticed a problem. If I am in the cruise at about 4400rpm the engine is smooth. When I throttle back it becomes rough and the roughness is noticeable on the rev counter with the needle oscillating instead of remaining steady. It is not a severe vibration but it is noticeable.

Sometimes the problem would go as quickly as it came. Other times it could be cured by applying power back to cruise setting.

More recently I have noticed that the vibration is not so quick to go and reapplying cruise power no longer fixes the problem.

I have looked through the various 'vibration' related threads and I see that there are a number of issues to look at. This is what I have looked at so far:


  1. Carb sync (which is good)
  2. Carb diaphrams for splits
  3. Carb pistons for sticking
  4. Throttle cables seem smooth
  5. Main needles
  6. Main jets
  7. Engine mounts


It does not seem to exhibit the problem when in the climb. It is more likely to be when off load, either slowing in the cruise or during descent.

I have not checked the prop balance or blade pitch. As far as I am aware nothing has been changed on the prop and the gearbox was serviced and reshimmed at 400hrs.

I might see if I can pick up the vibration using my iPhone microphone and trying to see the frequency of the vibration using one of the free spectrum analyser apps. That might tell me whether it is a prop blade problem or an engine problem.

Any other thoughts and suggestions would be gratefully received.

Regards,

Kevin

  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin,

    This sounds like an electric problem and not mechanical. You need to look at all your wires running from your ignition. You could have an either a loose connection somewhere or a damaged ignition wire which by the way is hard to detect.

    I would start out scouting and looking over all the wires for lose connection or faulty wire leading to and from your ignition system and then I would look at the ignition wires.

    I am almost sure that this is not a mechanical issue.

    Please keep us updated and good luck.

    Andre'
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Andre

    Why would it be an electrical fault if it seems to vary with load? Perhaps a high voltage problem might be load related but I cannot think of a mechanism that would link load to the low tension side of the ignition system.

    As far as I can tell from the Rotax service info there is no link between the ignition modules and any form of load sensing. Since the only input appears to be engine speed then I would imagine that my fault would occur at particular engine speeds rather than throttle positions/loads.

    I am happy to be proven wrong. In many ways an electrical problem may be easier to fix :)

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin,

    I'm with you and don't think this is electrical. Usually electrical is there or it isn't and usually doesn't change with rpm. Intermittent electrical issues are like chasing ghost. They're there when you don't want them and elusive when you are looking for them.

    Is the low rpm vibration issue repeatable each flight? If the rpm vibration is always down in the less than 2500 rpm range then suspect something in the idle jet which you should be able to confirm with a set of vacuum gauges, not an electronic one. You can't diagnosis very well with the electronic balancer's, but gauges will tell you several things and which carb.

    Here are a few things to rule in or out. First a double check on a carb sync at idle and up to at least 3500 and up to 4000 rpm, it may have changed. Check the throttle cable on the carbs and make sure both move fore and aft smoothly from idle to full throttle on the ground. I was going to say something about the gearbox, but it looks like it was done at 400 hrs., but maybe not done right? The Woodcomp prop should have the prop flange bolt torque checked and have a dynamic balance performed. Checking the prop blades for individual pitch against each other never hurts if you have not done so, but I don't think this has anything to do with your problem. Your engine mounts could be another cause if over 5 years old.

    On another note your engine would be happier in cruise at 5000 rpm or more and a WOT flat and level prop pitch setting to get over 5500 rpm instead of cruising in the mid to low 4000's.

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

      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Roger,

    The vibration is repeatable every flight. The engine seems to need to be up to full temperature and a reduction of power from 4400rpm to 4000rpm can be enough to bring on the fault. I have checked the carb sync at various power settings and and it is better than 0.25"Hg between sides.

    When the problem manifests itself I have switched off each ignition in turn and it makes no difference to the roughness. I deduce from this that the problem is either mechanical or fuel related. The prop is the only item that I haven't really looked at closely. I'll check the blade pitch and bolt torque but I think that a closer inspection of the throttle cables may be in order.

    It is a shame that there is no easy way to see the individual manifold pressures. I take it that flying with the balance tube disconnected and a couple of gauges fitted is not advisable.

    The 4800rpm maximum static rpm is dictated by our national aircraft approval body when using the Woodcomp prop so I cannot increase it. I am not sure of the rationale behind the limit, I would be interested to know if anyone has any ideas.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi,

    I would like to clarify from what you said that you synced the carbs at the rough running 4000rpm area ?

    I had rough running in the 3000- 4000 range I did a sync at 3500, verified good sync at 5000 . This solved my problem.

    One other thing I think possibly could help is dynamic prop balancing.

    do you know the current friction torque numbers of your gearbox ?
      
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