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Thread: Precision tuning the Rotax 912 UL/ULS

  1. #1

    Default Precision tuning the Rotax 912 UL/ULS

    For those that havenít followed the story on backcountrypilot, I figured Iíd post here - Bane is ALMOST back in the sky after some adventures in carb tuning over these past days.

    The data collection on this planes engine started about a week ago when I decided I was done hunting for the perfect idle and power settings per the rotax manual and forum suggestions, and I went back to the method Iíve used on every other engine - an air fuel meter and some time. The results of this installation and data collection showed that almost everything said about the 912 and Bing carbs on these forums to be complete nonsense. ďReturn it to 100% stockĒ leaves the engine at a ratio of 7.9:1 on the AFR. Instead of 1.5 turns out on idle mixture screw, 5/8 of a turn out placed is at 12.8:1 - the ideal ratio.

    secondly, needle in clip 3 position, mid range at 8.2:1 - move needle to 2nd clip position from top, arrive at around 11.1:1 on the ratio - rich but still better.

    lastly, main jet at wide open throttle runs 13.4ish:1 which is too lean. A bit of polish to clean it up today and Iím sure I can get it between 12.8 and 13.1.

    Moral of the story, tune your carbs to your engine. Iíve never in my life ran across an ďall inclusive ď carb setting that works for every engine the carb is bolted to. Sure, you can go buy a carb and turn some screws and get an engine to run, but that doesnít mean itís running right.

  2. #2
    Senior Member Eric Page's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    I think you'd be doing the carbureted 9-series engine community a great service if you were to write up your technique in detail. Include required equipment, how it's attached to the engine, what readings are important, which carburetor adjustments affect them, how to dial in each of the adjustments, and any part modifications you performed to get a good tune.

    A great many operators of these engines have zero experience with carburetor tuning (or engine tuning of any kind), so your experience would be very valuable. After reading just a few threads on this forum about carburetor tuning problems, I bit the bullet and spent the extra money for a 912iS. If I had found some solid guidance on tuning, I might have made a different decision and saved significant coin.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

  3. #3

    Default Re: Precision tuning the Rotax 912 UL/ULS

    Quote Originally Posted by Eric Page View Post
    I think you'd be doing the carbureted 9-series engine community a great service if you were to write up your technique in detail. Include required equipment, how it's attached to the engine, what readings are important, which carburetor adjustments affect them, how to dial in each of the adjustments, and any part modifications you performed to get a good tune.

    A great many operators of these engines have zero experience with carburetor tuning (or engine tuning of any kind), so your experience would be very valuable. After reading just a few threads on this forum about carburetor tuning problems, I bit the bullet and spent the extra money for a 912iS. If I had found some solid guidance on tuning, I might have made a different decision and saved significant coin.

    thank you for that feedback. Iím half considering making a short video that involves the tuning process as Iím seeing so little actual tuning info out there other than the blanket statements of turn it all back to stock.

    looks like I know what Iíll do with my afternoon today.

  4. #4

    Default Re: Precision tuning the Rotax 912 UL/ULS

    Ask and yee shall receive. Spent the afternoon after tucking her back into the hangar going over some of the details for yíall.


    https://youtu.be/GxEORpdJp0w

  5. #5
    Administrator DesertFox4's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    Thanks Nicholas for the video.👍


    DesertFox4
    Admin.
    7 Super Sport
    912 ULS Tri-gear


  6. #6

    Default Re: Precision tuning the Rotax 912 UL/ULS

    Glad to help! Today was another day spent tuning but managed to gain another 50 rpm with the tinkering. Officially hitting and holding 5400 on a prop that was previously only achieving 5200 on ground run up at the same pitch. This is promising that were able to pull a bit more power from these engines if we pay attention to the tuning.

    spoke with a Rotax certified mechanic today about what I was doing. He too had ZERO complaints with my methods as I discussed what all I had done - that right there makes me a bit happy.

  7. #7
    Senior Member Eric Page's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    Well done, sir! Thanks for posting your methods and results.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

  8. #8

    Default Re: Bane - Precision tuning the Rotax 912 UL/ULS

    no problem at all. I really hope some of yíall install an o2 sensor and check some readings. Iím half tempted to write a small arduino code so yíall can make a cheap display device as opposed to paying a few hundred for the setup Iíve got that way itís a cheap and easy means for yíall to monitor the condition of the engines fueling during flight.

  9. #9
    Senior Member 109JB's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    Not a 912, but I find that the Rotax specs for the 582 are also off a bit, particularly at low speed and mid range speeds. For the 582, the specs call for a 55 idle jet but with that it is slobbering rich and doesn't want to idle smooth. I don't remember what I have the air screw set for. I have a 40 idle jet in there now and it will idle right down to about 1800 rpm smoothly. Stock mid-range needle creates a lean spot. I changed the needle and got that cleaned up too. On the 2-stroke you really don't want to be lean in the mid to upper rpm range. The top end is pretty close on the 582 with the stock 165 jet. A bit rich, but works fine without fouling plugs and without burning pistons.

    Bottom line is that at least on the Rotax engines the stock jetting might not get you where you want to be. The air fuel ratio gauge is how I tuned my Sonerai with its VW engine. Worked well but still a bit on the rich side because some cylinders run a little leaner due to the intake runners being a little unequal length.

    BTW, there is a way to add mixture control to the Bing carbs on both the 2-strokes and the 4 stroke Rotax. If interested look up "Hackman mixture control" on the internet. I have a home brew version on my 2-stroke Kitfox and it works pretty well.
    John Brannen
    Morris, IL
    Sonerai IIL (Single Seat)
    Kitfox 3/4 1050 - Rotax 582 (Back Flying and sold)
    Kitfox IV 1050 - Rotax 582 (sold)
    Kitfox IV 1200 Speedster - Rotax 912 UL (project)
    Piper Twin Comanche (Sold)
    Glasair 1 FT (Waiting to start)

  10. #10
    Senior Member Delta Whisky's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    Quote Originally Posted by AvDES LLC View Post
    no problem at all. I really hope some of yíall install an o2 sensor and check some readings. Iím half tempted to write a small arduino code so yíall can make a cheap display device as opposed to paying a few hundred for the setup Iíve got that way itís a cheap and easy means for yíall to monitor the condition of the engines fueling during flight.
    Many thanks for the video - I'm interested in any details you can share (remember?) regarding the O2 sensor and the threaded bung or receptacle you used. As far as displaying the AF ratio - is there any chance a volt meter with a nearby conversion table could work? I'm also assuming that by using one O2 sensor in the middle of the muffler you made corresponding carb set ups and didn't worry about any minor differences between the left and right sides.

    Your timing couldn't be better for me. Maybe you saw in a different thread my exhaust stinger departed the aircraft during its last flight. There is some evidence it might have been related to insufficient clearance between the cowl and the exhaust stinger but there is also considerable evidence it was the result of a vibration issue. Without the capability of a thorough vibration test I'm committed to address both possibilities - and it won't hurt to make the engine as smooth as possible for comfort reasons. Anyhow, when the new muffler shows up, I'll be ready to add the O2 sensor if a reasonable ($$) way to interpret the output is available.

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