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  • Re: rough running 912ULS

    by » 8 months ago


    Thanks for the comment...this is the way I was advised to set it up when first building and I didn't really know better but I have also been a bit apprehensive about it since I have learned more for this exact reason.   The reason I have not changed it as of yet is the airframe is composite and it would mean having to remove the wings and either making a T junction with the outgoing fuel line from one of the tanks, which wouldn't allow much extra cooling if it is drawing fuel from that tank at the time or needing to drill a new hole in the tank and re-seal it.   Not impossible to do by any means but not a small job either.   Another option might be to add a small fuel cooling radiator to the circuit before returning to the gascolator.   Have you seen this done before or have an opinion on that setup?


  • Re: rough running 912ULS

    by » 7 months ago


    What Richard Said!  +++

    I see that you are in Australia and currently in the heat of summer.

    You are not so much returning hot fuel to the Gascolator, you are returning fuel vapor bubbles also.

    When the fuel exits the 0.02 orifice and the pressure suddenly drops to Zero, it "Foams" like when you open a soda bottle.
    It is not a great deal of vapor (Mostly Butane) but if it accumulates in the gascolator the only way out is going to be through the carbs.
    The vapors flow can favor one carb over the other resulting in one side going extremely lean.

    You do not want to Tee into the fuel line, you need to return the fuel and vapors back to the actual tank for venting.
    If your fuel tanks do not cross-feed, you will need to be sure to return the fuel to the tank it came from or risk overflowing the tank.
    The return line has about 5 gallons per hour of flow.  It is not a trivial amount.

    It is also possible to add a micro-sized header tank just for the purpose of seperating out the vapors.
    If there is an unused Out Port on your gascolator, it might also be possible to connect a vent line to it making the gascolator a micro-Header.


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


    Thank you said by: RotaxOwner Admin

  • Re: rough running 912ULS

    by » 7 months ago


    Flight Design and a few other company's do run the return line back to the gascolator and have been since 2004. 


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


  • Re: rough running 912ULS

    by » 7 months ago


    @Bill - is that 5 gallons or 5 litres please?  I've been trying to find a figure but gallons sounds a lot!

    @Roger - I can't find a reference but I'm pretty sure the return line on the CT was changed to going to the tank some years ago, the gascolator connection being found ineffective. I think it was a mandatory change at least here in the UK. The LAA won't accept return to the gascolator on any homebuilt aircraft that they deal with.


  • Re: rough running 912ULS

    by » 7 months ago


    Mike Wylde wrote:

    @Bill - is that 5 gallons or 5 litres please?  I've been trying to find a figure but gallons sounds a lot!

    @Roger - I can't find a reference but I'm pretty sure the return line on the CT was changed to going to the tank some years ago, the gascolator connection being found ineffective. I think it was a mandatory change at least here in the UK. The LAA won't accept return to the gascolator on any homebuilt aircraft that they deal with.

    The new CT's using the 912iS engine run the return line back to the header tank. All the ones before using the 912ULS ran it into the gascolator until the header tanks came out.


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


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