Carb Icing

Category: 912 / 914 Technical Questions 4 months ago
Hi all, Can anyone on the forum explain how carb icing manifest’s itself on a 912 UL, the other day I had what I would call a misfire pulse that shook the whole aircraft then cleared, about3 or 4 minutes later it did the same again, it did this several times then started running OK.
I have looked at the carbs for debris in the float boats and all OK.
Any ideas or was it just carb icing.
Andy.
( no carb heater fitted)
  • Re: Carb Icing

    by » 4 months ago


    Carb ice is caused by the cooling produced as the fuel is atomized in the carburetor.
    If the ambient temperature and dew point are within range, the cooling will cause the air in the carburetor to cool below the dew point and the interior walls of the carb will "Sweat".
    If the temp and manifold pressure are low enough, the now liquid water will freeze and layer upon layer of ice will begin to build up eventually stopping the engine.
    Under the proper conditions, carb ice can happen at ambient temperatures up to 100°F/40°C.

    Counterintuitively, High humidity and Low temperature are Not conducive to icing.
    Highly Humid Air contains so much water that it is difficult to cause it to freeze.
    Low-temperature air does NOT contain much water vapor.

    https://www.boldmethod.com/learn-to-fly/systems/carb-ice/

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

    Thank you said by: RotaxOwner Admin

  • Re: Carb Icing

    by » 4 months ago


    Bill, thanks for your reply, I fully understand the causes of icing in the manifold, what I was after was how it affects the 912 engine, I was trying to work out if ice falling from the manifold and into the engine would cause the vibration pulse I was getting every now and then for about 5 or so minutes.
    Thanks Andy

  • Re: Carb Icing

    by » 4 months ago


    I would think that carb Ice would be more persistent than one miss every few minutes.

    What fuel including history, RPM, and MAP were you running.
    Kind of sounds like an intermittent "Knock".
    Any chance you still have some "Winter Blend" Gas in it?

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

  • Re: Carb Icing

    by » 4 months ago


    I agree
    Try checking your ignition system, mainly the rotatory ignition key if you have one installed.

  • Re: Carb Icing

    by » 4 months ago


    Hi, I think I’ve found the problem although I don’t know why it should make it misfire,
    I decided to change the plugs on pulling one of the bottom plug leads off I found it was just resting over the plug and not pushed home. It must have been making contact most of the time but not all, Not sure why this would cause a misfire as it should have carried on firing on the second plug, any thoughts on why the misfire.

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