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I just got back from accompanying a friend interested in purchasing a 12-year old aircraft with a 912ULS.  On preflight I tried to check the oil level and the engine would NOT burp, even after many prop revolutions.  The owner said it never would burp for him, or for the previous owner/builder.  So, what could be the reason why this engine, during its 12-year life, will not burp?  After my numerous attempts, the oil level on the dip stick was mid way up the flat portion of the stick.  Anyone know why?

  • Re: Engine Won’t Burp

    by » 3 months ago


    Usually failure to burp is due to spinning the prop too fast. As you turn the prop, pause for a second or two at the top of each compression stroke to allow the compression gases to pressurize the crankcase. It usually takes me about 10 to 20 compression cycles to burp depending on temp, use, etc.


  • Re: Engine Won’t Burp

    by » 3 months ago


    Mine usually takes far more blades to get to burp than I want to admit myself. I’ve gotten into habit of just checking the oil level after each flight as it’s fairly easy to do at that point. I’m sure it’s taken me over 50 blades on occasion to get my to burp.  I’ve owned the airplane for 2 1/2 years and never had a problem


  • Re: Engine Won’t Burp

    by » 3 months ago


    Compressions are good?  Should burp with blow-by from pistons as you hand turn engine.  Blow-by pressurizes the crankcase and sends oil to remote tank.  The burp sound is when last bit of oil goes into tank and compressed air then exits the crankcase.  


  • Re: Engine Won’t Burp

    by » 3 months ago


    Bob your friends Rotax 9 has a problem.

    Rotax 9 oil circulation is in large part dependent on piston "blow- by" to raise the level of pressure in the crank case that in tern forces oil into the reservoir/tank. (From the tank, a pump draws the oil back into the engine & reticulates the oil though/to the moving parts.) If this system is not working as designed;

    The engine is likely to experience accelerated wear.

    The pilot can not make a reasonable assessment of pre flight oil level.

    I would speculate that;

    The oil tank is incorrectly located - See Rotax installation manual for recommended tank to engine relationship.

    Low compression - have a compression "Leak Down" test done.

    As speculated above, your friend is not conducting the "Burp" process using an effective technique. I know of several Rotax owners who find the Burp process onerous and often skip doing it - I always council against this attitude.


  • Re: Engine Won’t Burp

    by » 3 months ago


    Thanks for the suggestions, guys.  I’ll say right now he didn’t purchase the aircraft for other reasons, but as the owner of an RV-12 myself, but with a 912iS, my curiosity was aroused.  So, I’m quite familiar with the burping process and theory. The oil tank is in a proven position relative to the engine, the engine only has 160 hours on it - yes, 160 hours over 12 years! - so, I ruled out compression issues. And having never burped in over 12 years by multiple pilots, and pulling the prop through myself a good 20 times, very slowly, without burping, I was at a loss for an explanation.  I always check the oil after flight on mine, but since I was going to be the test pilot on an aircraft I had not maintained myself, I wanted to check oil level before taking to the sky.  So, with that extra color, any other thoughts?


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