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Greetings:

I have a 2015 912ULS on my RV-12, total time about 160hrs.  At this years annual condition inspection, among other work perfomed, I complied with the needle valve circlip SB.  The carburetors were balanced and the post inspection engine run was fine.

About 2 hours (and a couple of flights) after the condition inspection, the engine quit as I was touching down.  A quick restart, and I taxiied back to the hangar.  I did a runup before shutting down and everything seemed fine.  The next time I went to fly I could not get the engine to run at idle without (at least) some choke.  When warm, below 2000 rpm, it runs very rough (if at all) and the RH EGT drops to nothing.  I pulled the #3 cyclinder spark plugs (same cylinder as the EGT probe and the plugs where definitely whitish - the appearance of an over-lean condition.

I have reset the idle mixture on the RH carburetor and pulled the needle valve to make sure the circlip did not break.  I am now out of ideas.  Thoughts, anyone?


Jerry Allison

2016 RV-12

X39 - Lutz, FL

  • Re: 912ULS Won't Idle Without Choke

    by » one month ago


    Make sure the mixture screw is at 1.5 turns out. This sounds like your idle jet(s) are clogged. Easy to remove and clean. Call me if you need help. Roger @ 520-574-1080 in Tucson, AZ 


    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-574-1080 Home (TRY HOME FIRST)
    520-349-7056 Cell

  • Re: 912ULS Won't Idle Without Choke

    by » one month ago


    Thank you, Roger.  I also removed the idle mixture needle valve.  It appeared to be clear (I could shine a light through).  When I put the needle valve back in, I went in all the way and then backed it out 1 and 1/2 turns.  Is there another jet that I need to inspect?  I will call you later today to discuss.  Thanks again!


    Jerry Allison

    2016 RV-12

    X39 - Lutz, FL


  • Re: 912ULS Won't Idle Without Choke

    by » one month ago


    Have a look at this article: 

    https://electricmotorglider.com/2017/04/11/bing-64-cv-carburetor-idle-circuit-part-3/

    "If the engine runs with the choke partially on, but dies as the choke is placed in the off position, it is an indication that the idle circuit is the culprit. It is absolutely essential that the idle circuit be set up and functioning properly. We use the idle circuit on every flight, and it is a surprisingly important system within the carburetor.  Aside from the practical aspects of having a properly operating idle circuit, there are many correlations with the idle circuit malfunctioning and other engine problems, ranging from increased maintenance to engine stoppage and even engine failure."

    Seems to describe your situation exactly.  Did you have a look at the two bypass holes?


  • Re: 912ULS Won't Idle Without Choke

    by » one month ago


    Thank you again Roger and Todd.  An update:

    The right idle jet was clear.  I ran it again after reassembly, same problem.  So now, obviously, I'm suspecting the left carb as the culprit.  But why the disparity in EGT?

    So I first troubleshoot the EGT and find they are indeed swapped - at the back of the Dynon unit.  Near term, I swapped spade connectors at the engine so that I have correct indications (this conflicts with wire markings on the engine harness, but now I'm sure the left probe is indicating Left, and the right probe Right on the D180).

    Then I removed the left idle jet and found it clogged - with what, I don't know. The bowl was clean, no debris in the bottom (thankfully).  I carefully tried to unplug the idle jet with a nice, soft strand of copper wire (very thin) onto a white piece of paper so that I could tell what the debris was. No luck.  I carefully blew it out but still was unable to capture the debris.  When I reassembled everything, it now idles much better, almost correct (1700 rpm when warm) - although a little rough (see below) The carbs do still need to be balanced. And both sets of floats weighed 6.4 grams, well under the 7 gram limit for float sinking concerns.

    Now I have a significantly higher drop on B ignition, 250 rpm and it is a little rough compared to 150 rpm dorp on A, relatively smooth.  I'm sure my spark plug gap (new plugs at annual condition inspection) is set at .025".  I was meticulous about this. All the plug wires are seated good.  Next step will be to pull, clean and test each spark plug.  That has to wait for another day.


    Jerry Allison

    2016 RV-12

    X39 - Lutz, FL


    Thank you said by: RotaxOwner Admin

  • Re: 912ULS Won't Idle Without Choke

    by » one month ago


    Jerry,

    you will find the time to sync the carbs well invested. Since you found the cause and fixed it syncronising the carbs will most likely eliminate the remaining roughness. It does make a difference.


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