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912 uls idle mixture and balance

in General Tech Discussion 6 years ago
I have a 912 uls with 800 hrs. checking carb balance at 4000 rpm is spot on but as rpm lowers balance falls off. At 2000 it is off by a good bit. Does one adjust the idle mixture screws to balance low rpm?
Thanks

  • Re: 912 uls idle mixture and balance

    by » 6 years ago

    No you adjust the cables and or linkage. They pretty much self balance once the throttle plates are opened up it is in the lower rpm's that they need precise synchronization.
    Thank you said by: James Dernovsek
      
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  • Re: 912 uls idle mixture and balance

    by » 6 years ago

    Sync the carbs between 1600-1800 with the idle stop screw. If you do set the carb sync for idle down below 1800 you can do that with no issues so long as you remember to not sit and idle there on the ground. Use the lower rpm setting for shutting down the engine or final approach. Due to some windmilling the prop will not turn that slow on approach as it would on the ground. This isn't done with the Bowden cable adjustment. The Bowden cable adjuster is used for the higher rpm sync. The book says 2500 I far prefer 3300-3500. Your 4k wasn't an issue. You are also correct that if you sync let's say at 2500 many times the needles will split again is throttle is increased. This is why I prefer 3300-3500 sync vs 2500 as the book states.
    Remember that in low rpms you are in the idle circuit and jets and at high rpms you leave that and are using the needle and main jet. This is more representative for a carb sync at higher rpms to get out of the idle circuit. Plus not all cables and throttle behave perfectly from the idle setting to higher rpms so it should at least be looked at.

    I know of only one company that tries to set carb sync by backing out the idle stop screws all the way and only syncs the carbs at 2500. Doing this is not correct and the carbs have to be off somewhere if this is how you do it. With your own results showing a difference between 4K sync and out of sync at idle you just proved that point. The cross over balance tube helps the carbs in the low rpms, but it is not a cure all.

    Sync your carbs properly at at every 100 hr. and or annual inspection. Once done right doing it at the next inspection only takes a few minutes.

    Be good to your engine. It needs to live, run and be happy for a couple thousand hours.

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

      
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  • Re: 912 uls idle mixture and balance

    by » 6 years ago

    ScreenShot2019-03-14at07.27.57.png (You do not have access to download this file.)
      
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  • Re: 912 uls idle mixture and balance

    by » 6 years ago

    Eugene, I've been flying behind a homemade version of this for years now. Nothing not to like about it!

    Alan
    Thank you said by: Eugene Klimenko
      
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  • Re: 912 uls idle mixture and balance

    by » 6 years ago

    Here is my homemade balancing tube. I did it last year with very good results.
    IMG_0997.jpeg (You do not have access to download this file.)
    IMG_1009.jpeg (You do not have access to download this file.)
    IMG_1008.jpeg (You do not have access to download this file.)
      
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