Precision tuning the Rotax 912 UL/ULS

[trond]

doubt if you really want to send anything to here. It is a long way to Stavanger
Will check fluid levels. Might be low as you suggest
12.5 mm or 1/2 inch is the magic number ?

[AvDES LLC]

I wish I could confirm that 1/2” was the magic number, but if I set mine to that, both of my carbs leak fuel out of the atmospheric pressure tubes on the sides of the carbs. I’ve had to tinker with mine to get them to where they are at the point of any further adjustment would cause leaking and leave it there. Maybe someone with the shock monster gear and 29’s (my setup) can confirm they too had to do this?

[trond]

an update and a question; I changed up needle jet from 2.72, to 2.74, 2.76, then to 2.80 and with 2.80 it runs perfect at cruise AFR just below 15 which is good. WOT still at 12.5 witch is great too and at 2000 AFR 13. However with the 2.80 and needles in richest position it got too rich at 3000 RPM and AFR 10 with misfires. I had not expected the needle jet to affect mixture that low in the register? I lowered needles back to next richest clip position 2 from bottom but still too rich at 3000 at misfires, then to next leanest position (clip 2nd position from top), I also leaned out on the idle mixture screws by 1/2 turn (now less than 1 turn out) and re-syncronised. It still runs rich AFR 10-10.5 at 3000 RPM but no misfires. I guess maybe that doesn't matter as engine spends next to no time at 3000 and AFRs and behaviour everywhere else is fine.....

Any advise ? Smaller idle jets and get screws back out to 1.5 turns ? What do you think ?

[AvDES LLC]

You're officially running into the same issues that I did when it came ot tuning the mid range of these engines to the bing carbs - my take on that situation, return it to whichever settings get you the best AFR in the idle and low range as well as the top end. If you look at a few of my graphs youll see that I too end up in that rich setting at the mid-band and nothing I tinkered with would make it go away. Due to the fact you spend so little time at those RPM, I ended up writing it off as the region of diminishing returns and left it be. As long as that rich condition doesnt end up causing bogging as you pull power for landing you should be good. You can test this with a high speed taxi down the runway at full throttle, then slowly pull power as if your coming in on landing and note the engines response - this will ensure the most safety as youre still on the ground but getting high RPM, non-static information out of the system.

[trond]

thinking my issues may have to do with the lack of an airbox. CV carbs like to have an airbox which help to get the vacuum needed to lift the slide. One trick used on bikes with CV carbs and modified or no airbox is to shorten the spring to allow the slide and needle to rise higher. Clearly the lean at cruise issue can not be resolved with needle jet alone. It is possible to get it to run fine at cruise with a larger jet needle, but then it will run stupid rich further down in the RPM range (2400-3400 in my case). If I use the smaller needle jet to make it run OK at 2400-3400 then it will be very lean at cruise with AFR above 15 and misfires. I see no easy way to resolve the issue as there is no room for an airbox. Only two options: either another needle or shorten the spring, Or injection obviously but that is too expensive for me. Any thoughts ?

[jrevens]

This is a very interesting thread, and being educated as an engineer I appreciate the fine-tuning & experimentation. One comment though (just for the heck of it)... why so much effort to fix something that isn't broken? To clarify, I'm based at KBJC in Colorado with a field elevation of approx. 5700 ft. I've had my 912 ULS up to 18,000' and I've flown the Kitfox down to OSH 3 times. I've never had any misfiring or other important issues yet, and fuel burn is within Rotax specs. Obviously it runs a little rich at the higher altitudes, but it's smooth, no missing, & seems to be fine to me. Spark plugs are cheap enough to just replace when necessary, although I haven't yet experienced any problems from the plugs. Granted, I don't have much time on my engine yet (200+ hrs.) but my buddy Stan, in a hangar across from me, has had the same experience with his, and by the way - he replaced his original 912 with a new one after running it for 2400 hrs. with no major repair/maintenance. And it was a 1200 hr. TBO engine! It was still running OK, but it was a prudent move IMO... he sold the old one to Hal Stockman. Have many others had any real problems with proper operation and the original factory set-up? Is it a misconception on my part that people seem to be having more problems after they start changing things? Or are we just lucky at this point?

[AvDES LLC]

thinking my issues may have to do with the lack of an airbox. CV carbs like to have an airbox which help to get the vacuum needed to lift the slide. One trick used on bikes with CV carbs and modified or no airbox is to shorten the spring to allow the slide and needle to rise higher. Clearly the lean at cruise issue can not be resolved with needle jet alone. It is possible to get it to run fine at cruise with a larger jet needle, but then it will run stupid rich further down in the RPM range (2400-3400 in my case). If I use the smaller needle jet to make it run OK at 2400-3400 then it will be very lean at cruise with AFR above 15 and misfires. I see no easy way to resolve the issue as there is no room for an airbox. Only two options: either another needle or shorten the spring, Or injection obviously but that is too expensive for me. Any thoughts ?

When addressing the mid range, see my last post. I too ran into this issue with a rich condition at the mid range and left it as is due to the engine spending so little time in this RPM range. Remember a rich condition can cause less than desirable traits - bogging, low power, all the way down to a complete shut down if its too rich - but if I recall correctly mine is running in that 10.5:1 range in the 3500 RPM area as well and have had zero issue or bogging. Lean conditions destroy engines - at the end of the day the atomized fuel brought into the cylinder during the intake stroke does actually cool a cylinders walls. When you reduce the amount of fuel to the point that it becomes that lean, you're removing cooling from the cylinders. Couple this with extreme cylinder temps in lean conditions and you're creating a case for serious disaster.

This is a very interesting thread, and being educated as an engineer I appreciate the fine-tuning & experimentation. One comment though (just for the heck of it)... why so much effort to fix something that isn't broken? To clarify, I'm based at KBJC in Colorado with a field elevation of approx. 5700 ft. I've had my 912 ULS up to 18,000' and I've flown the Kitfox down to OSH 3 times. I've never had any misfiring or other important issues yet, and fuel burn is within Rotax specs. Obviously it runs a little rich at the higher altitudes, but it's smooth, no missing, & seems to be fine to me. Spark plugs are cheap enough to just replace when necessary, although I haven't yet experienced any problems from the plugs. Granted, I don't have much time on my engine yet (200+ hrs.) but my buddy Stan, in a hangar across from me, has had the same experience with his, and by the way - he replaced his original 912 with a new one after running it for 2400 hrs. with no major repair/maintenance. And it was a 1200 hr. TBO engine! It was still running OK, but it was a prudent move IMO... he sold the old one to Hal Stockman. Have many others had any real problems with proper operation and the original factory set-up? Is it a misconception on my part that people seem to be having more problems after they start changing things? Or are we just lucky at this point?

Depends on what you consider the definition of "broken" to be. If you scroll all the way back to the start of this thread ive included many videos of the steps made to get to current. In those videos, I talk about the original numbers that came out of the stock carbs upon wiring an O2 sensor into my system - from memory I believe my WOT AFR ratio was somewhere in the 14.7:1 ratio. These numbers were taken after a complete carb rebuild so this is "best case" values from all new parts, o-rings, jets, etc etc. During the tuning process and again cited in the videos, I had three separate instances where small adjustments to tuning to bring the AFR into a more desired range (12.8 - 13.1) yielded me an increase of 300 RPM. To date through tuning alone I have had a total gain of 900 rpm and as such have had to increase the pitch on the prop to bring the engine back down to reasonable RPM at full throttle. This has resulted in the prop taking a bigger bite out of the air (obviously), a reduction in EGT's (increase pitch = reduced EGT), and best of all, my little 80 HP 912UL keeps up with any ULS I have flown next to. For a first hand accounting, anyone feel free to reach out to Kevin Palmer and he can confirm these claims - his plane has wing strut fairings and a 100 HP ULS. I have no strut fairings and a UL and keep up with him no problem. Lastly, as a result of the increase in RPM and cruise speed, I pull my throttle back to cruise at about 5150 RPM and my fuel flow meter indicates 5.1 GPH at cruise. All of the data collected during this effort has supported that while they arnt "broken", these carbs are also not tuned to these engines as best as they could be.

[Delta Whisky]

This is a very interesting thread, and being educated as an engineer I appreciate the fine-tuning & experimentation. One comment though (just for the heck of it)... why so much effort to fix something that isn't broken? To clarify, I'm based at KBJC in Colorado with a field elevation of approx. 5700 ft. I've had my 912 ULS up to 18,000' and I've flown the Kitfox down to OSH 3 times. I've never had any misfiring or other important issues yet, and fuel burn is within Rotax specs. Obviously it runs a little rich at the higher altitudes, but it's smooth, no missing, & seems to be fine to me. Spark plugs are cheap enough to just replace when necessary, although I haven't yet experienced any problems from the plugs. Granted, I don't have much time on my engine yet (200+ hrs.) but my buddy Stan, in a hangar across from me, has had the same experience with his, and by the way - he replaced his original 912 with a new one after running it for 2400 hrs. with no major repair/maintenance. And it was a 1200 hr. TBO engine! It was still running OK, but it was a prudent move IMO... he sold the old one to Hal Stockman. Have many others had any real problems with proper operation and the original factory set-up? Is it a misconception on my part that people seem to be having more problems after they start changing things? Or are we just lucky at this point?

John - you make (as usual) really good points. I will add though that I installed an AFR gauge just to see how well the carbs on my plane did their job and (as written elsewhere in this forum) was very pleased to see how well they did it. The one point that made me rationalize that the effort was worth it (besides the enjoyment of learning via experimentation) was discovery that at idle the mixture was way too rich. AND - setting it to a much leaner position drastically improved smoothness - certainly at idle and slightly above. One point to make - as more and more time has accumulated on the engine the AFR has appeared to be getting leaner and leaner. I'm using 100LL and am very suspicious that lead is driving the readings and not the carbs. From this experience I recommend decisions to make mixture adjustment(s) NOT be made using anything but new probes. I'm still pursuing more opportunities (experiments?) to improve engine performance (smoothness anyhow) and am accumulating the parts to install a larger cross over tube. More news at 11:00 and, as usual, your mileage may differ.

[AvDES LLC]

And this is why I love experimental aviation. As long as we’re smart about it, we can make HUGE strides. I look forward to your updates on the crossover upgrade!

John, agree with Delta - you always add to the conversation and I’m pleased to have you participating. If you have the older style slip fit exhaust and not the ball and socket setup, I have fabricated a muffler with AFR bungs welded in that is a simple pop and swap. I’ll gladly mail it to you with an AFR meter and you can see where your engine is at with the current carb settings - no modifications required. PM me if you’re interested in taking the setup for a whirl and providing some data driven input.

[jrevens]

And this is why I love experimental aviation. As long as we’re smart about it, we can make HUGE strides. I look forward to your updates on the crossover upgrade!

John, agree with Delta - you always add to the conversation and I’m pleased to have you participating. If you have the older style slip fit exhaust and not the ball and socket setup, I have fabricated a muffler with AFR bungs welded in that is a simple pop and swap. I’ll gladly mail it to you with an AFR meter and you can see where your engine is at with the current carb settings - no modifications required. PM me if you’re interested in taking the setup for a whirl and providing some data driven input.

Thanks Nicholas - I appreciate the kind words and the offer but I think I'll pass on that for now. I'm getting too lazy in my older age and don't want to mess with something that's working well for me now. I'd like to have good cockpit adjustable mixture control, and have thought about the HACman retrofit but don't know too much about it. Incidentally, I've thought a little about using an oxygen sensor with the Rotax for a few years. I actually designed & built a simple voltage comparator circuit back in the early '90s that I used with the Lycoming in my Thorp T-18 for about 20 of the 28 years that I flew it. I welded a bung on my nice SS cross-over exhaust system for the inexpensive '60s era 2-wire oxygen sensor I used to sense when the proper stoichiometric mixture was reached, indicated by a red light on the panel. There was a similar system made by someone in Canada that was sold by Aircraft Spruce. It was very quick & accurate. AFR meters weren't as readily available for a reasonable price back then. 100LL sure limited the life of those sensors, and I'd change them out at about 200 hrs. Just a little ancient history.