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While doing several short (Young Eagles) flights, our RV-12 (912ULS) started running very rough at 3100 RPM, and would not go any faster. The engine runs fine at idle and the carbs were in proper balance. Per the universal 012 mantra, "check the carbs", I found a carb float bowl gasket that was bunged up. On replacement, the engine started running normally. Reviewing the RotaxOwner carb rebuild videos, there was a short comment to the effect "check that the gaskets are placed properly, or the engine may run rough".
Well, the problem is solved, but I just wanted some confirmation of the symptom/fix from collective wisdom of all, and maybe an explanation: With a leak in the float bowl gasket, I'm assuming this either 1) messes up the fuel level in the carb bowl, starving one side of the engine or 2) messes up the pressure differential in the CV mechanism (which I don't fully understand).

As I said, the problem appears to be solved, I just need confirmation that the leaky gasket is a viable cause of the problem.
  • Re: Rough running engine--carb float bowl gasket probl

    by » 6 years ago


    If the leak was significant then yes it would cause problems. It would change the balance between cars, it would change the sync and it could suck air and or leak fuel all over the hot exhaust.
    Did you check both float bowls for debris? Did you replace the bunged up gasket?

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


    Thank you said by: Robert Reese

  • Re: Rough running engine--carb float bowl gasket probl

    by » 6 years ago


    Yes--checked both carbs float bowls--both seemed very clean. I replaced the errant gasket with positive results. At idle and up to about 3100 RPM, things ran smooth, but it got rough and maxed out right about that range. I didn't observe any gas leakage--so I'm not sure if the gasket is holding the gas in or the air out.

  • Re: Rough running engine--carb float bowl gasket probl

    by » 6 years ago


    I was hoping it was an easy carb bowl check with some floating debris. You may still have a blockage in a fuel line after the pump. It may be in the banjo bolt on top of the balance tube that feeds both carbs, the attachment point at the carb or something in the carb that wasn't in the bowl.

    You may also want to check the fuel pump for pressure on the outlet side.

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


  • Re: Rough running engine--carb float bowl gasket probl

    by » 6 years ago


    Your engine is now running fine right ? If NO then...

    Any work done before the problem started ?

    It sure seems like a restriction in the fuel supply somewhere.

  • Re: Rough running engine--carb float bowl gasket probl

    by » 6 years ago


    My first thoughts also were that one side had some fuel restriction problems--some booger finally shook loose and clogged the works up somewhere, since things were running fine for three short flights, and then the sudden onset of problems.

    I unfortunately didn't methodically change one thing and test the results. I did some of the least-likely-to-be-the-problem things first:
    1) Checked the gascolator bowl/filter screen--pristine.
    2) Checked the plugs--no problems there.
    3) The fuel pressure on the Dynon Skyview was always fine, but that is measured off a point common to both carburetors.
    4) I removed and checked the fuel lines feeding each carb (the original banjo bolt-based distribution, which was having some leaking problems, was earlier replaced with AN fitting-based fuel lines, so there were no tight restrictions as with banjo bolts-based fuel systems).
    5)Then I replaced the bad carb float bowl gasket, before then firing up the engine and finding the problem solved.

    So I guess the root cause could remain a mystery--though I could re-install a bunged-up float bowl gasket to observe the effect--problem being that the effect might very well involve some undesired pyrotechnics.

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