Precision tuning the Rotax 912 UL/ULS

[AvDES LLC]

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


https://youtu.be/GxEORpdJp0w

[GuppyWN]

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


https://youtu.be/GxEORpdJp0w

Awesome video. I admit I'm pretty ignorant on carb tuning. What do you use to find the fuel/air ratio while testing?

[AvDES LLC]

thank you very much.

for AFR info I developed a split exhaust that allows for tuning of the carbs independently. Pm for more info if you’d like so I don’t clog up this thread with a diatribe.

[Delta Whisky]

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!!)

[AvDES LLC]

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!!

[Delta Whisky]

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.

[AvDES LLC]

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.

[trond]

needle is already in richest position

[trond]

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

[Delta Whisky]

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