Precision tuning the Rotax 912 UL/ULS

[AvDES LLC]

https://www.summitracing.com/parts/avm-30-0300

heres the correct one. It comes with everything you’ll need including the weld bung and sensors.

and did install and initial run up of the dual monitoring system. I’ll do a write up but a short video is on Instagram name: AvDES.LLC

[bbs428]

Thanks!

[AvDES LLC]

https://youtu.be/UCMQPFdUtmY

Newest tuning video is up!!!

[MarkH]

Have you seen any correlation between AFR and EGT?

[jrevens]

About 30 years ago I basically built a simple voltage comparator circuit and mounted a common ‘60s era oxygen sensor on the hottest cylinder’s exhaust pipe of my O-320 Lycoming. It used a small red LED on the panel right in front of my face. I adjusted it to light the LED at the air/fuel ratio when the EGT peaked. There was a similar commercial version available at the time. I also had a single probe Alcor EGT gauge & probe for comparison. Biggest advantages over EGT were basically instant reaction, with the red light in my field of view giving me the indication. I was adjusting my mixture much more frequently, for instance when climbing or descending. I could adjust my vernier mixture control so that the light would flicker on and off when very close to peak, then steady at peak & over. So, I would adjust so that the light just went off (or it could drive me crazy). I think it was one of the primary reasons that my fuel burn consistently was lower than every other 160 hp O-320 powered T-18 that I ever compared numbers with. The biggest disadvantage was that the lead in 100LL would pollute the sensor and it would have to be replaced about every 200 hours. At least they were relatively inexpensive.

[AvDES LLC]

Have you seen any correlation between AFR and EGT?

100% there is a correlation. Egts will increase as you approach a more lean condition. What the egts don’t tell you is the combustion quality. I tried tinkering with the engine with just EGT and brought them down to around 1300 at cruise flight by tinkering with idle fuel and pitch, but no further info could be gained and no further adjustment made due to the lack of information available with this method. Upon installing the AFR probes, I was seeing those very lean conditions that would have never been noticed had I not started down this rabbit hole. Sure, the egts were within spec, but the engine was also running at 14.7:1 which is not a good thing.

I do think that the age old approach of tinkering via Egt output is great when it comes to engines with in flight mixture control, however if you are flying with these Bing carbs, you cannot make in air adjustment and as such the time needs to be spent on the ground tuning the carbs to the engine so that at altitude no adjustment is necessary.

[MarkH]

Agreed, using the EGTs is a blunt instrument and AFR is superior.

We have an aircraft which is based on the Kitfox IV / Avidfox Flyer with a 912 UL and 95hp Xtra kit.

When we bought it the carbs were setup with 158 main jets and the needle circlip in position 2. The engine ran well until the carbs were serviced by an AME who recommeded they be returned to stock. Since then, in my opinion, it runs rich, which is evident from both the fuel flow and spark plug colour.

As a start we installed EGT probes on cylinders 3 and 4. I don't have cruise EGTs yet but the peak EGT is around 740 degrees Celsius (1364 F) which is well below the 1472 F Rotax spec (the EGT probes are mounted 70 mm from the exhaust flange, as this is a late 1990's motor).

The plan is to change back to the original setup, with the needle circlip in position 2. From what I have read, this should result in an EGT increase of 50F, and cleaner burning plugs (more optimal AFR). In the absence of an AFR gauge, the EGT and plugs are the only feedback I have.

Our field is at 4780ft and only run 95 mogas.

Your observation, and measurements, confirm that Rotax 'stock' setup is excessively rich and not suitable for all conditions.

[AvDES LLC]

well its great to hear were at least getting consistent results when returning to the "stock" setup. Im interested to hear some numbers if you ever decide to temp install a AFR meter for fine tuning.

[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.