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I have a 914 in a kitfox that I bought last year.  When I bought the airplane the fuel pumps were connected in parallel with no check valves.  I was seeing wide variations in differential fuel pressure readings when I switched from 1 to 2 pumps on.  The most troubling things was that at mid-low power the motor would begin running rough within seconds of having both fuel pumps on.  I had no issues running one fuel pump at any power setting and running both pumps seemed to be fine at high power settings.

I have since re-plumbed the fuel pumps so they are in series with check valves in accordance with Rotax's latest instructions.  I also replaced the fuel pressure regulator at the advice of a certified rotax repair shop.  I initially thought this had solved my rough running problem at low- mid range power.  However I've noticed that now it takes a couple of minutes (rather than 20-30 seconds) before the engine begins to run rough.  The fastest resolution seems to be when I shut off both fuel pumps for a second or two.  The engine almost instantly roars back to smooth running power and then I switch on a single fuel pump and it continues running normally.  Again the issue never arises with just one pump on (at any power setting) or at high power settings it runs fine with both pumps on.

It seems to me that I am flooding the carbs, but I thought the pressure regulator should address that.

I should also mention that I rebuilt the carbs about 20 hours ago due to a rough mid range.  That seemed to resolve that issue quite well.

  • Re: 914 Runs Rough with Both Fuel Pumps On

    by » one month ago


     

    Did you weigh your floats?


  • Re: 914 Runs Rough with Both Fuel Pumps On

    by » one month ago


    It sounds like the Float Valve is not sealing completely.

    There would be three causes of this.

    1. Your Floats are Heavy as Mr. R suggested.

    2. The Float Valve itself is not closing completely.

    3. Your fuel pressure is higher than the float valve can overcome.

     

    Your problem is actually a combination of all three.

    The floats are not pushing hard enough for the Valve to overcome the pressure.

    When you turn ON both pumps the pressure increases just enough to cause the problem.

    Saying the fuel pressure too great for the closing force is just as valid as saying the closing force is too low for the fuel pressure.

    Measure the fuel pressure. Weigh the Floats. Inspect the float valve for debris or damage.


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

    Thank you said by: Paul Whetstone

  • Re: 914 Runs Rough with Both Fuel Pumps On

    by » one month ago


    Thanks for your responses.  I weighed the floats about 15 hours ago and they were under 7 grams.  I’m hoping pressure is within limits given that I have a new fuel pressure regulator, but definitely worth double checking.  I suspect #2 (float valve debris or damage) is the biggest contributing factor.  I’ll give the float valve a more careful inspection.  Thanks again for the help.


    Thank you said by: RotaxOwner Admin

  • Re: 914 Runs Rough with Both Fuel Pumps On

    by » one month ago


    The motor only has 80 hours on it but it sat for several years before I bought the plane.  I wonder if the rubber tip on the float valve could have hardened up a bit while it sat for so long and now doesn’t seat perfectly?


  • Re: 914 Runs Rough with Both Fuel Pumps On

    by » one month ago


    If the plane sat for years, the carb bowls most likely dried out.

    Gasoline does not evaporate completely and your valve may very well have a coating of "Varnish" on it preventing it from sealing.

    Are you running MoGas?  Running a tank of E-10 autogas will help dissolve a lot of the "Varnish" and clean up the fuel system.

    You mentioned that you recently Re-Built the Carbs.  Wasn't the valve replaced at that time?

    - - -

    If you weighted the Floats 15 hours ago; that means you did it yesterday!

    The floats are soaking in the gasoline 24hours a day whether the engine is running or not.


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

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