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Thread: Precision tuning the Rotax 912 UL/ULS

  1. #61

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    Default Re: Precision tuning the Rotax 912 UL/ULS

    finally got the lambda sensor hooked up and went for a spin today. 912 UL with base settings, except needle in richest position. Measured AFRs, sonde on outlet so measures both banks together
    - between 12.5/13 when taxiing so idle circuit is good
    - 2000-4000 same, on decent/no load around 13-14
    - 4000 and up on cruise it leans out and at 4800 I have AFR15
    - 4800-5300 straight and level 15.5, which explains the less than perfect running. Pull out choke and get 14.5 and clean running
    - full throttle , including full throttle climb, I see 12.5-13 to it seems main jet is OK too


    So I think I need a larger needle jet. I have a 2.72s

    New ignition leads, caps and modules
    individual KN air filter pods
    All carb parts replaced
    Perfect leakdown numbers on all four
    220 hrs total on engine

    Any advice, or comments would be welcome

  2. #62
    Senior Member Delta Whisky's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    Depending on where your needle is, you might want to try raising it a slot first. Quick, easy and doesn't cost a thing.

  3. #63

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    Default Re: Precision tuning the Rotax 912 UL/ULS

    needle is already in richest position

  4. #64

    Default Re: Precision tuning the Rotax 912 UL/ULS

    Quote Originally Posted by trond View Post
    finally got the lambda sensor hooked up and went for a spin today. 912 UL with base settings, except needle in richest position. Measured AFRs, sonde on outlet so measures both banks together
    - between 12.5/13 when taxiing so idle circuit is good
    - 2000-4000 same, on decent/no load around 13-14
    - 4000 and up on cruise it leans out and at 4800 I have AFR15
    - 4800-5300 straight and level 15.5, which explains the less than perfect running. Pull out choke and get 14.5 and clean running
    - full throttle , including full throttle climb, I see 12.5-13 to it seems main jet is OK too


    So I think I need a larger needle jet. I have a 2.72s

    New ignition leads, caps and modules
    individual KN air filter pods
    All carb parts replaced
    Perfect leakdown numbers on all four
    220 hrs total on engine

    Any advice, or comments would be welcome
    with the lean condition present at the higher RPM range I would bet money on your mains and not your needle jet. As stated my Delta, ensure the needle is at richest position and if so, I’d step the mains up first. Remember that when you tune carbs, you typically tune at wide open throttle first and then DOWN the rpm range, not up. With your lean considering being in the 4K and up range, that’s where you’re carbs are relying on the main jet as it’s metering jet, not the needle. Once you open up that main, you’ll likey have to bring clip position back down so you’re sitting in that sweet spot during mid range. Just my .02.

  5. #65

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    Default Re: Precision tuning the Rotax 912 UL/ULS

    here is a good read on Bing tuning. Basically a CV carburettor (constant velocity) does not behave same as a normal slide carburettor. It is the load/manifold pressure that dictates slide&needle position, and not the RPM or throttle setting (obviously at WOT there will be high load too unless diving).
    The fact that I get 12.5-13 at WOT makes me believe I have the correct main jets. I have already richened up the needles to the btm position
    If I go to larger jets I will be running too rich at full throttle/load settings. I think

    At cruise it is the needle jet that is most important, not the mains

    But I take your advice and have added larger mains to the order from Bing just in case I should be wrong ! Hopefully they will arrive in a week or two and I will post an update



    http://jabiru.no/content/documents/m...ft_Engines.pdf

    Climb out: Since the amount of fuel delivered to the engine at throttle settings of ¾
    open or more are controlled by the main jet, we make a change in the main jet to come
    closer to our target of 1225. If climb our EGT’s are higher than target the mixture is too
    lean and a larger main jet is indicated. If EGT’s are lower than target the mixture is a bit
    rich and a smaller main jet should be tried. Remove the main jet to see what size is in
    the engine. Main jet sizes are a three digit number and most likely 255 for a 3300 and
    245 for a 2200. Adjust up or down by 5: ie 255 to a 250 or 250 to 255.
    2. Cruise: At settings of ¾ throttle or below the limiting factor for fuel delivery is the
    needle jet. Cruise flight should be at settings at or below ¾ throttle in most cases. The
    target for the center of the range for cruise flight throttle settings is 1325 F. Based on
    your EGT observations in cruise flight install a larger needle jet if temps are above the
    target or a smaller needle jet if temps are below target. Needle jet sizes are numbered
    in x.xx format. You may find a 2.85 needle jet in your carburetor and will need to try
    perhaps a 2.88 if temps are a little high or a 2.90 jet if temps are quite high. If cruise
    temps are too low then a smaller needle jet is indicated

    Note that the above numbers are for Jabiru with Bing carburettor
    Last edited by trond; 11-21-2022 at 01:27 AM.

  6. #66

    Default Re: Precision tuning the Rotax 912 UL/ULS

    Quote Originally Posted by AvDES LLC View Post
    with the lean condition present at the higher RPM range I would bet money on your mains and not your needle jet. As stated my Delta, ensure the needle is at richest position and if so, I’d step the mains up first. Remember that when you tune carbs, you typically tune at wide open throttle first and then DOWN the rpm range, not up. With your lean considering being in the 4K and up range, that’s where you’re carbs are relying on the main jet as it’s metering jet, not the needle. Once you open up that main, you’ll likey have to bring clip position back down so you’re sitting in that sweet spot during mid range. Just my .02.
    oh crap I read that wrong - you are 100% correct. I thought I read that you were getting lean at 4K and staying lean throughout the rest of the range. Re-reading it, you’re dead on the money at the upper end and as such my entire previous statement went in the crapper. Shouldn’t read posts while sitting in the shop trying to focus apparently.

  7. #67

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    Default Re: Precision tuning the Rotax 912 UL/ULS

    I have had an issue with lean mixture and vibration at cruise and ordered some larger needle jets from Bing. Today I tested with 1 step larger needle jets, up from stock 2.72 to 2.74. I did not see much difference on the AFR in cruise, if any. Mixture was perfect 13-13.5 from 2000 up, and in climb after takeoff 12.5-13 which is good for power. But when I level off and throttle back at 5100-5400 I see AFR around 15.5. It felt like engine was running smoother though.



    So I landed and changed up one more step, to 2.76. Minimal difference. AFR at cruise 15.2-15.5. If I pitch nose up AFR drops to 14.5. Needles already at richest position. Spark plugs look nice and grey/tan with clean insulator down to the root.






    One thing I have noticed is at lean misfiring the AFR shoots right up, with the unburned gasoline in the exhaust. So it is possible to get fooled by false readings. And maybe what I read now with less misfiring are more solid numbers ?


    Don’t know how to proceed. Maybe leave as is and remove AFR meter. Maybe order even larger needle jets. I only increased by 2 points which may be very little ? Oil in the KN filters ?

    In my motorcycle days one trick to get the CV carburettor to work on the KTM 640 without a proper airbox was to cut shorter the spring inside the throttle slide....



    • 11.5 AFR - Best Rich Torque at Wide Open Throttle (WOT)
    • 12.2 AFR - Safe Best Power at Wide Open Throttle (WOT)
    • 13.3 AFR - Lean Best Torque
    • 14.6 AFR - Stoichimometirc Air/Fuel Ratio Value (Stoich)
    • 15.5 AFR - Lean Cruise
    • 16.5 AFR - Usual Best Economy
    • 18 AFR - Carbureted Lean Burn Limit
    Last edited by trond; 12-10-2022 at 10:14 AM.

  8. #68

    Default Re: Precision tuning the Rotax 912 UL/ULS

    when performing full power run up on the ground what are you seeing for AFR? Have you ran a wire through your main jets yet at any point during disassembly for the needle jet exchanges?

  9. #69

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    Default Re: Precision tuning the Rotax 912 UL/ULS

    I see 12.5 -13 on full power static. Same as on WOT climb. It is only when load is reduced it goes lean. I have decided that only 2 steps up was too fine steps and will order more needle jets. Should be here in a month... meanwhile I can at least fly as it runs better already. Thanks for inspiring me to install the AFR. I use them on bikes and cares but for some reason I did not occur to me on my own that I should have one in the plane. I really would have liked a split exhaust to do it properly but L & R spark plugs read similar so I will use the AFR as an indicator only (as it should be) to see where changes happen and how large the changes are. And double check with plug readings

    Main jets are new and very clean. Inspected them today even

  10. #70

    Default Re: Precision tuning the Rotax 912 UL/ULS

    Hmmm. Those static numbers seem pretty good. Have you verified your float level? You’re be surprised just how much the level of those floats actually impacts AFR.

    if you’re every wanting to tune with a split exhaust we offer the one I’ve developed for rent to other 9xx series engine owners. Please drop me a PM if you’d like to discuss independently tuning each bank.

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