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Hello,

I am checking an ultralight 2012 912 uls with only 154 hours.

The compression tests shows 78/80 on cylinder 1 and 2. But 68/80 on 3 and 4. The owner usually fly once a month or just make a runup. I suppose is the rings but the engine is very "new", is it normal because the low usage?

Anybody has some experience about it?

Best regards.

  • Re: Low compression low hours 912uls

    by » one month ago


    First the basics...

    Was the Engine still warm from the Pre-Test Runup?

    Was each Cylinders at Top-Dead-Center when measured?

    Fly it again for 30 minutes and retest.

     


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


  • Re: Low compression low hours 912uls

    by » 6 weeks ago


    Bill Hertzel wrote:

    First the basics...

    Was the Engine still warm from the Pre-Test Runup?

    Was each Cylinders at Top-Dead-Center when measured?

    Fly it again for 30 minutes and retest.

     

    Hi Bill,

    Yes, we warm before the test, and we repeat the test, warm again and test again immediately after it.

    Yes, each cilinder in center. We note air noise on oil tank.

    So, if possible I will test again.


  • Re: Low compression low hours 912uls

    by » 6 weeks ago


    Are you saying you are determining top-dead-center from the burp noise in the oil tank??


  • Re: Low compression low hours 912uls

    by » 6 weeks ago


    Tyler Hathaway wrote:

    Are you saying you are determining top-dead-center from the burp noise in the oil tank??

    No Tyler,

    Just we had burp noise on test.

    Is it possivel top-dead-center error, making burp noise in the oil tank?

    The mechanic did many tries and the cylinder 1 and 2 he got 78/80, it's is strange he make mistake in other cylinders.

    I remember that he put the finger on hole of spark plug, when there was pressure out he stopped. He used a screwdriver to mensure the cylinder position too.


  • Re: Low compression low hours 912uls

    by » 6 weeks ago


    You are over thinking this.

    When you "Burp" the engine you are compressing the air in the cylinder and it is bypassing through the piston rings into the crankcase and venting into the Oil taknk, making the sound.

    When you are performing the compression test, you are introducing Compressed Air into the cylinder and it is bypassing through the piston rings into the crankcase and venting into the Oil taknk, making the same sound.

    In Both case the piston is at or near Top-Dead-Center (TDC).

    The prop is held at TDC to prevent it from rotating because  it is a lot easier to hold the piston at TDC than it would be to try and hold it half-way Up, and the compressed air will blow the piston to the bottom if allowed to do so.

    If the piston is at the bottom, one of the valves will start to open and no compression testing will be possible.

    TDC on the Compression stroke is the Only place you can get a reading.

    If you accidently found TDC on the exhaust stroke, the valves would be open.  No Compression!  0 / 80.

     

    All of this is perfectly normal and to be expected.

     


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


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