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compression seems low on one cylinder

in General Tech Discussion 10 years ago
I HAVE A 912UL, when turning the prop prior to checking the oil I find one cylinder has very low resistance as though the compression is low.
Motor doesn't use any oil.
Will only rev to 5200rpm
Any suggestions?
Regards
Brian.

  • Re: compression seems low on one cylinder

    by » 10 years ago

    Hi Brian,

    Is this something new? Do a compression leak down test on all cylinders. Did the engine used to get more than 5200 and is the prop over pitched?

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

      
    Report

  • Re: compression seems low on one cylinder

    by » 10 years ago

    Thanks for reply.
    I will get a leak down test done.
    I've tried changing the prop pitch but no more adjustment.
      
    Report

  • Re: compression seems low on one cylinder

    by » 10 years ago

    Was this something that happened all of a sudden out of the blue and the engine was running fine up to this point? How many hours on the engine?

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

      
    Report

  • Re: compression seems low on one cylinder

    by » 10 years ago

    Don't want to hijack this post, just add to it and hopefully get some advice myself....I have had a similar problem on my 912 uls. #4 cylinder was trending lower over a 6 month period, eventually less than 60/80 psi, with the differential leak down test showing air rushing into the oil tank with the lid off, and no air escaping through either the inlet or exhaust manifolds. All other cylinders 75/80, engine approx 410 hours since new. I suspected piston rings, however upon disassembly found that the ring end gaps had near aligned. As I had the new rings, I fitted them with the ring gaps in accordance with the heavy maintenance manual, only to find after the first engine run that the compression was still low, same symptoms. Upon 2nd disassembly, to my surprise I discovered the ring gaps had realigned themselves. I aligned the rings as per the heavy maintenance manual, ran the engine again and same low compression symptoms upon differential test....I can only assume that the rings have once again moved.

    Any ideas from the group? I'm at a loss right now....
    JJ
      
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  • Re: compression seems low on one cylinder

    by » 10 years ago

    Sorry Jeremy, the following is more comment than help.

    I might be wrong but I would have thought that from a static perspective (such as a leak down test) the position of the ring ends won't make much difference.

    In a dynamic situation such as when the engine is running then I imagine that not having the rings aligned would reduce the amount of combustion gas bypassing the rings.

    I'm not sure why the rings would rotate and align when the engine is running.

    Kevin
      
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