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Thread: Precision tuning the Rotax 912 UL/ULS

  1. #51
    Senior Member Delta Whisky's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    Nicholas - this input is very preliminary but today was the big day following completion of the condition inspection, installation of a new muffler and of the AFR gage. I only had time to idle around the hangar complex and get the engine up to temperature; the report is: at low speeds, just a tad above idle, the AFR was 10.45. I have a little work to do on the cowl before a flight but if all goes well, I hope to have in flight numbers tomorrow after improving the idle AFR. Thanks for getting this approach to tuning on my radar screen. (Sure hope it works for me!!)

  2. #52

    Default Re: Precision tuning the Rotax 912 UL/ULS

    This is great to hear! You’re already way better than my initial conditions of 7.4 at anything below 2300 roughly. Did you get a chance to let the engine come to full temp and do a full power run up to see what you were getting for AFR at “takeoff” condition? If not, I would strongly recommend it before flying. With the AFR meter hooked up you can get a very good idea of what your plane will do in all rpm ranges without ever having to leave the ground, cut the engine, dead stick it in, and pull the plugs. Those days are over baby!!

  3. #53
    Senior Member Delta Whisky's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    Well, the truth of the matter is - I can't remember to bring my data sheet home, from any of my last two flights, but I don't want to wait any longer to share what I've found. Bottom line the idle was, as noted earlier a bit rich. With the idle mixture screw now set at one turn off of closed it is 11. 5 to 12.2 (it varies a bit - nothing is steady with an instrument that offers 2 digits to the right of the decimal point). That's fine by me but I will set it again and attempt to find the magic setting that will give somewhere between 12.5 and 14 at idle. The real news is I can't imagine how a compensating carburetor could do any better than what I saw over 3 hours of cross country with altitudes ranging from 1,500 to 10,500. The actual values moved a bit from the high 11's to a few times close to 12.8 but if I were to estimate an average, it would be a tad above 12.

  4. #54

    Default Re: Precision tuning the Rotax 912 UL/ULS

    great to hear you got some serious time in the plane and we’re able to observe the system throughout the duration. By chance what are your AFR numbers at around 1800 rpm “idle”? I ask as this will clue you into the condition of the actual idle circuit and will involve less tinkering with the screw. If at 1800 ish you’re running noticeably rich (11.8 or below would be my personal range) I would consider changing out that idle jet and trying again. I know I stated that my screws are basically 1/3 of a turn out from fully seated, but only thing I take away from that is that I need to bring my idle jet down from a 35 to probably a 30.

  5. #55
    Senior Member Delta Whisky's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    I finally remembered to bring the data sheet home and plot said data. It wasn't exactly what I remembered when including the altitudes above my normal flight altitudes. It does reveal that under about 7500 feet the AFR range is a very reasonable 11.8 to 12.8, is very linear, and and makes me want to believe that if you are after a condition tuned for your expected altitude range it should be possible. Because I seldom have reason to fly above 4,000 feet, I'm going to leave well enough alone. But - I do have an additional, cool looking gauge that should make my passengers marvel even more at how challenging it is to fly a plane.

    AFR data graph.jpg

  6. #56

    Default Re: Precision tuning the Rotax 912 UL/ULS

    outstanding results! I met with another gentleman this week that took the tuning kit home and pulled baselines from his engine as well. From what I’ve seen so far his carbs are doing great in the 2300 and above ranges with an almost linear output similar to yours. Looks like some of these carbs are on point, others (mine) were drastically off. I’ll post up his numbers as well once we get numbers through the entire engine rpm range. Seems that those of you closer to sea level are having way better results than those of us a bit higher up

  7. #57

    Default Re: Precision tuning the Rotax 912 UL/ULS

    Well Bane got a new top end and as a result some monitoring was in order for the AFR as I knew there would be something new to see due to all of the recent changes. Here is the most recent results with the carbs left as they were while running the UL top end ( take those 912UL top end numbers from before with a grain of salt as there was so much blow-by im surprised the thing even ran). With that said, a new run of tuning is about to begin once I get my dual exhaust setup back later this week. For now, single bank AFR readings are all were going to get:
    Attached Images Attached Images

  8. #58
    Senior Member Delta Whisky's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    UPDATE - I finally got around to making another adjustment on the idle mixture screws to get the AFR closer to optimum prior to a long cross country. The idle smoothness and throttle response improved just a bit more. Satisfied . . . . . until landing for fuel at a small airport on a long cross country the engine quit while on final. It started up right away so it really wasn't a big deal. Turns out that the now leaner mixture setting without a necessary change to the idle speed setting was the cause of the problem. (Turned out that with just a bit of extra pull on the throttle you could make it quit.) Now with the necessary and corresponding settings made all is well. Lesson learned and shared.

  9. #59

    Default Re: Precision tuning the Rotax 912 UL/ULS

    glad to hear the story and the lesson and not a news article. Happy you got it sorted! How are you liking the engine performance post- tuning as opposed to the prior performance? Any input on changes in cruise temps in regards to cht's, egt’s, coolant temps, etc?

  10. #60
    Senior Member Delta Whisky's Avatar
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    Default Re: Precision tuning the Rotax 912 UL/ULS

    To be honest, because I saw a "need" to adjust only the idle circuit, I haven't paid any attention to the inflight parameters.

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