Precision tuning the Rotax 912 UL/ULS

[AvDES LLC]

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.

[Eric Page]

Well done, sir! Thanks for posting your methods and results.

[AvDES LLC]

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

[109JB]

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.

[Delta Whisky]

no problem at all. I really hope some of y’all install an o2 sensor and check some readings. I’m half tempted to write a small arduino code so y’all can make a cheap display device as opposed to paying a few hundred for the setup I’ve got that way it’s a cheap and easy means for y’all 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.

[AvDES LLC]

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.

Glad it helped! As far as the bung, you are correct in the assumption that it was centrally located to gain an "average" understanding of what was happening between the two sides and all adjustments were mirrored from one side to the other. In a perfect world. we would pull the muffler, connect a single downpipe to each side and have an AFR meter for each bank of cylinders. If I had the free time and extra funds I would make that happen, but I own an airplane... so broke as a joke haha.

As far as interpreting the output, there should be a way with a multimeter. Youll receive a signal ranging between 0 and 5V. You could linearly extrapolate the points between these voltages and correlate them to a AFR rather easily. I believe 0V is roughly 7.4 and 5V would equate to about 22.4 - I can verify these numbers for you if youd like.

In the coming weeks when I finally align and complete a few more projects ill be taking a stab at that arduino code for an AFR meter. That would be a rather small investment for everyone to make and would be a much faster means to collect data than comparing a dynamic value to a data table.

Please post up any of your progress on this and any questions. Id love to actively participate in your tuning while posting in this thread so people are able to see and read through your process in real time - I got too frustrated chasing the rabbit hole trying to figure this all out the first time to make a video that wasnt comprised of mainly curse words.

109JB, thank you for your input on this as well. Glad to see that others are noticing the same thing I am and that our data supports each others findings - overly rich in idle was a major root cause to the excessive vibrations I had at low RPM. Correcting that idle circuit had a HUGE impact on the rest of the engine tuning and starting there is 100% what I recommend to anyone that is going to be moving forward with this.

Now, looking at this thread title, I do not think its clear the information that is being shared in this thread - anyone have any suggestions as to a better thread title to get this information out to as many 912 folks as we can?