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912UL Engine vibration on throttle back

in 912 / 914 Technical Questions 12 years ago

  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    The Rotax data for the Rans S6 with 912 UL (80hp) is third groove from the top. It took some determination to even dare move my port side carb. needle on notch weaker to #2 down after repeatedly observing cooler by 100 deg.C EGT on that side.

    Kevin.
    One: Is your second groove up/Rotax position 3, one notch weaker than the Rans Rotax manual ?

    Two: My roughness/engine was actually cessation of power at ~ 4,500 cruise regime. Below 4,000 is a speed I rarely use so there could be a few trembly patches on the way down the rpm range. {Nowadays following Lee's advice here I run the engine harder which seems better all round. It means the carb. is well into main/needle jet territory. I'm surprised your minor adjustments to the idle setting can really be significant that high up the rpm's.

    However at <4,000 with windmilling effect too, you should be right down on carb. throttle position, in which case low/slow running settings apply. But not to IMHO change the plug colouring which should come from in flight use unless you spend ages shutting down ?

    Perhaps your fault is a rather different affair felt at lower power demands than my rough/engine failure to provide cruise power ?

    mike hallam [England].
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin,

    I just read Mike's post. Do make sure your carb needle clips are in the #3 position. That is one up from the bottom slot on the needle. If you are running it in the #2 slot it will be running a little lean especially in cold dry air. I don't think carb cleaning spray will work to check for an air leak if you choose that method. You may need starting fluid. It is ether and highly volatile. It does work and you would need to spray everything one at a time starting at the carb socket and then the air intakes on top of the cylinders to see if you have an "O" ring leak. I really good tip. Just in case keep a fire extinguisher at hand if you choose this method. I haven't seen a fire, but I don't like to take chances. If the rpms increase when you spray one spot at a time you have found your leak if it is a leak. Just another thing to rule in or out.

    It can be frustrating to find a pesky little problem sometimes.

    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

    Mike,

    The info on the needle setting came from the generic Rotax Heavy Maintenance manual 73-00-00 page 28 Ed.1 Rev2. (see attached extract from the manual).



    This setting has always previously given me the book figures on fuel consumption of around 13 litres/hour at 90mph.

    Given that my roughness is at rpm below cruise power it is quite possible that idle mixture adjustment could have a significant effect.

    Given the local forecast I cannot see myself attending to this any further until after Christmas now.

    Kevin
    rotax912ULneedleposition.jpg (You do not have access to download this file.)
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    O.K. Kevin, my book says exactly the same. the picture is more secure than slot counting.

    mike hallam.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Ahhhh, this is doing my head in. It doesn't appear to be binding throttle cables. The cables move freely, carbs re-balanced. I have pulled right carb off and checked float bowl and levels and ok as noted that right front spark plugs black, all others ok. Compressions all feel fine. Can't reproduce problem on ground, occurs everytime I fly, with throttle back vibration starts, very noticeable. If left alone, it now gets worse to point of not wanting to let it continue. Only way of stopping it is to close throttle and advance open again. Recurs as soon as close throttle from high power settings though, like after climb. Can occur in any rev range from 5000 down to 4200. Fuel consumption seems ok, has been averaging only 11 litres/hobbs hr. Only thing I can think that is different is I installed some carb drip trays, had the O ring on the intake manifold to drip tray joint with no O ring between drip tray and rubber carb mount. Maybe an airleak there? Have not changed any factory settings to the carb otherwise. When flown this morning, went well for about 20 mins or so then the problem started. Other thing that is doing my head in, the plastic bush at lower atttachment point of throttle spring has now split on the right carb. Left carb still ok as was only recently replaced after spring cut through it and the bracket with subsequent forced landing at idle power! This motor has only 80 hrs, how long are those plastic bushes meant to last??

    Frustratingly

    Paul
      
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