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Sudden partial loss of compression

in 912 / 914 Technical Questions 12 years ago
Hello,

Preambule:
1. installed a thermostat and purged the oil system as per instruction video.
2. synched the carbs
3. installed new spark plugs
4. since then 10 hours of flight. All systems OK. (oil pressure 3.0-3.2 bar....).
5. ROTAX912 80hp. About 200 hours.

I have flown yesterday and both my wife and I said what a smooth running engine. :)

This morning when hand propping I noticed a strange partial loss in compression :sick: This alerted me. :ohmy:
Started the engine and stopped immediately because it sounded as if the engine was not running on all cylinders. :sick:

I have checked the tightness of all spark plugs: all are OK. Oil level OK.

This afternoon I will check the compression per cylinder as a verification to what I noticed but I am now very puzzled what could be the cause. :(

Any ideas of what could be the cause would be welcome as pilots hate to be grounded!

Cheers Jan

  • Re: Sudden partial loss of compression

    by » 12 years ago

    Hi Jan,

    Is it possible that the problem is a perceived problem and not a real one? You'll need to do a little more checking first.

    You'll need to see if this is an issue from your install, electrical or fuel issue.
    What was the second set of differential compression test numbers? Is one really bad? If one cyl. is bad it may be possible your oil purge didn't work out right and you had a large bit of air still in the system or the thermostat is in wrong or has a problem?

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

      
    Report

  • Re: Sudden partial loss of compression

    by » 12 years ago

    Hi Roger,

    Just back from the airfield. All compressions OK. Test run engine OK.

    So could it be this:
    When turning the propellor to "massage" the engine sometimes a lack of compression of certain cylinders may be observed due to sticky valves.

    My experience:
    1. Turned the prop: one or two hard spots due to compression. Rest nil.........abnormal.
    2. Started the engine. Cut the engine due to relative violent shaking.
    Later.
    3. Measured the compression of the 4 cylinders: OK!
    4. Turning the prop again: normal behavior!
    5. Testrun of the engine: OK!

    Did I eliminate sticky valves due to the starting attempt earlier the day such that normal compressions are back again or is it something else?

    Cheers Jan
      
    Report

  • Re: Sudden partial loss of compression

    by » 12 years ago

    Hi Jan and Roger,

    My 912ULS makes the samething as you describe. The problem is intermitent. Before each start I turn the prop by hands and sometimes one cylinder doesn't have any compression. I know if I started the engine in this condition it will shake badly. When I notice that, I start the engine with both magneto off for a couple of seconds and it seems do release the sticking valve. Then I start my engine as usual. I still don't understand why it happens. My 912ULS has 400 hrs and is running great with all temps in spec. In fact, it is running almost perfect as I started to have some problems with my sprag clutch on start up.

    Hope someone may help us with this weird engine behavior...

    Pat

    P.S. By the way, do you folks think it's normal for an engine rated with a 2000hrs TBO to be removed from an aircraft after only 400hrs for a faulty and MISDESIGNED sprag clutch. This will cost me 2000$ to make it fixed. Sorry, I have hard time to contain my frustration with this issue.
      
    Report

  • Re: Sudden partial loss of compression

    by » 12 years ago

    Sprag's usually get damaged by rotating the prop backward or too many kick backs on starts and stops. They usually last for a long time.
    $2K for a sprag replacement is awful high.

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

      
    Report

  • Re: Sudden partial loss of compression

    by » 12 years ago

    Hi Roger and Pat,

    The reason of this behavior is that it may show up when the engine is cold. :cheer:

    I found out that it is not necessary to hand prop before starting the engine. The compressions have to be measured anyway when the engine is warm to moderately hot. Further, you have to have a good synch at 1600-2200 rpm to reduce violently shaking when the engine is starting.

    Cheers Jan
      
    Report
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