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Annoying Rotax 912ULS engine vibration

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lgold(at)quantum-associat
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Posted: Sat Mar 15, 2008 12:59 pm Post subject: Annoying Rotax 912ULS engine vibration

I need  help solving a Rotax 912 ULS (100-hp) vibration problem. 

For the  first 20 hours of flying my new Z-701 there was no noticeable vibration. When I  changed the Warpdrive prop pitch at 20 hours total time (on both plane and  engine), the vibration started. It vibrates at all RPMs except at idle, where it  seems the same as it was before. It’s not extreme, but I won’t fly my 701 until  it’s fixed.  

I’m  reasonably certain that the problem is not related to resetting the prop. The  prop is new. I changed the pitch from 12° to 11.4° very carefully, just as I did  initially (when there was no vibration problem). After the vibration was noted I  reset the pitch back where it was but the engine still vibrates. I reset it  3-times and am certain the pitch of each of the 3-blades and the way they track  are very accurate (variation about .1° and 1mm), and the blades are seated  properly. Because of this I feel the problem just happen to occur at the same  time I changed the pitch. 

So far,  to try to solve this problem, I have changes the spark plugs (they all look  good), re-sync’d the carbs, cleaned the fuel filters, cleaned the main, and idle  jets (no dirt), checked the cylinder pressures (all 180#), checked to make  certain that the choke and throttle cables were set properly and are not binding  (they are OK), and examined the engine mounts (look OK). Switching mags does not  change the amount of vibration. 

I asked  for help from the tech at Lockwood (who is a great resource). He thought the  vibes may be caused by a dirty idle jet (said this is a common new-engine  problem). I also went to the Rotax-engine Matronics group and checked out or  eliminated almost all their suggestions (sticky choke cables, etc.). The two  recommendation I did not check-out were: (1) Bad gas: (I use 91 Octane Chevron  MoGas and no one else on the field who uses this stuff has reported problems),  and (2) A “sprag clutch circlip” coming undone (or breaking) and going through  the sprag clutch gears. This came from someone who had vibration and found the slipper  clutch had been incorrectly installed. He said there are three disk springs that  must oppose each other; his were installed in the same plane, which eventually  damaged the sprag clutch so the engine would not start. (I am looking at the  simplest solutions first. This issue  probably resulted from a rebuild. Since my engine only has few hours on it, my  vibes are probably not related.) 

 Any suggestions to help  resolve this vibration problem, from 912’ers who have had similar experiences,  would be welcome. 

Les

   [quote][b]

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Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat Mar 15, 2008 3:51 pm Post subject: Re: Annoying Rotax 912ULS engine vibration

static and dynamic balance absolutely has to be ruled out.

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Roger Lee
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lgold(at)quantum-associat
Guest

Posted: Thu Mar 20, 2008 9:53 am Post subject: Annoying Rotax 912ULS engine vibration

Roger,

The vibration problem was not due to prop balance. It was a combination of

slightly loose bolts that mount into the engine and  a slightly off carb

sync. The mechanic told me that the old style Zenith engine mounts will

loosen up because the engine isn't support right. Unless you add mounts on

top of the engine these bolts need serious attention. He suggested changing

the four 10mm bolts to studs and using lock washers, which he said won't

loosen.

Les  



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