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No compression on one cylinder when burping

in 912 / 914 Technical Questions 12 years ago
Today when burping my engine during preflight I had no or very low compression on one of the cylinders. The best way I can describe it is it felt mushy instead of the normal pressure/resistance. It still gurgled normally. I started it, ran beautifully, static load test was fine. After running the engine compressions were back to normal when rotating by hand.

912 ULS, 170 hours total, annual about 2 months and 10 hours ago with all cylinders 86/87. I've owned the plane for two years, always burp, and have never felt this before.

Does anyone have any insight into what may have caused this and if it is an indication of a problem? Thanks

  • Re: No compression on one cylinder when burping

    by » 12 years ago

    Hi, I don't have an answer for you, I am curious as to what the more experienced members say on this.

    For a relaxed mind , I think if you are concerned of your engine compression, I would do another compression test. I have my own differential tester so , I think I would just do another compression check, just for piece of mind.

    After saying that..., you mentioned the engine ran flawless after this . I would think something should stand out in temps or power if there was something seriously wrong.
      
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  • Re: No compression on one cylinder when burping

    by » 12 years ago

    Could it be possible that this was the air that we do the burping for? causing the different feel.

    I burp mine on every start and have never really paid too much attention to the feel of each compression stroke , just wait to hear the gurgle.

    I use the slow burp method , it reduces the amount of turns to get the gurgle, as well if you burp it after shutdown, it will take less turns to gurgle the next time you start.

    Kevin
    912 uls 285hrs
    Kitfox 5
      
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  • Re: No compression on one cylinder when burping

    by » 12 years ago

    Thanks Kevin. Everything still felt right today so I went up. Flew for over two hours, everything was normal.

    When this happened yesterday, it wasn't just a minor difference in feel, almost like there was nothing there at all at that point. No way to miss it. I can't say I have ever paid that much attention to the "sameness" of the compression strokes either. But will now...

    My Rotax guy is going to refresher trainiing next week and is going to ask about this. If he learns anything I will post it here.
      
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  • Re: No compression on one cylinder when burping

    by » 12 years ago

    Had a similar thing on mine about ten years ago. If my memory serves me correctly we thought it had something to do with turning the engine backwards and a hydraulic tappet losing oil. After a ground run it was back to normal. I have never turned the engine backwards since and have never had the problem again.
      
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  • Re: No compression on one cylinder when burping

    by » 12 years ago

    All piston rings have gaps. That's how they can be replaced on the piston. All rings will move. Occasionally and randomly the gaps line up. When they do you get low compression. I would never tear down a cylinder based on one low compression reading. I would go fly a few more hours and do another compression check to see if the compression improves. If it does, as it did in this case, I would ignore it and attribute it to the ring gaps aligning.
      
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