Precision tuning the Rotax 912 UL/ULS

[AvDES LLC]

For those that haven’t followed the story on backcountrypilot, I figured I’d post here - Bane is ALMOST back in the sky after some adventures in carb tuning over these past days.

The data collection on this planes engine started about a week ago when I decided I was done hunting for the perfect idle and power settings per the rotax manual and forum suggestions, and I went back to the method I’ve used on every other engine - an air fuel meter and some time. The results of this installation and data collection showed that almost everything said about the 912 and Bing carbs on these forums to be complete nonsense. “Return it to 100% stock” leaves the engine at a ratio of 7.9:1 on the AFR. Instead of 1.5 turns out on idle mixture screw, 5/8 of a turn out placed is at 12.8:1 - the ideal ratio.

secondly, needle in clip 3 position, mid range at 8.2:1 - move needle to 2nd clip position from top, arrive at around 11.1:1 on the ratio - rich but still better.

lastly, main jet at wide open throttle runs 13.4ish:1 which is too lean. A bit of polish to clean it up today and I’m sure I can get it between 12.8 and 13.1.

Moral of the story, tune your carbs to your engine. I’ve never in my life ran across an “all inclusive “ carb setting that works for every engine the carb is bolted to. Sure, you can go buy a carb and turn some screws and get an engine to run, but that doesn’t mean it’s running right.

[Eric Page]

I think you'd be doing the carbureted 9-series engine community a great service if you were to write up your technique in detail. Include required equipment, how it's attached to the engine, what readings are important, which carburetor adjustments affect them, how to dial in each of the adjustments, and any part modifications you performed to get a good tune.

A great many operators of these engines have zero experience with carburetor tuning (or engine tuning of any kind), so your experience would be very valuable. After reading just a few threads on this forum about carburetor tuning problems, I bit the bullet and spent the extra money for a 912iS. If I had found some solid guidance on tuning, I might have made a different decision and saved significant coin.

[AvDES LLC]

I think you'd be doing the carbureted 9-series engine community a great service if you were to write up your technique in detail. Include required equipment, how it's attached to the engine, what readings are important, which carburetor adjustments affect them, how to dial in each of the adjustments, and any part modifications you performed to get a good tune.

A great many operators of these engines have zero experience with carburetor tuning (or engine tuning of any kind), so your experience would be very valuable. After reading just a few threads on this forum about carburetor tuning problems, I bit the bullet and spent the extra money for a 912iS. If I had found some solid guidance on tuning, I might have made a different decision and saved significant coin.


thank you for that feedback. I’m half considering making a short video that involves the tuning process as I’m seeing so little actual tuning info out there other than the blanket statements of turn it all back to stock.

looks like I know what I’ll do with my afternoon today.

[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

[DesertFox4]

Thanks Nicholas for the video.👍

[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?

[DesertFox4]

For more clarity on the content of this thread, I can go back and change the title on all posts but have to do them one by one so let me know soon what you think is a more concise title before too many more posts.
I did add some generic “tags” below for easier searches in the future. Anyone can add more “tags”at anytime that will help members in the future find this in-depth thread on precision tuning a Rotax carb equipped engine.

[AvDES LLC]

For more clarity on the content of this thread, I can go back and change the title on all posts but have to do them one by one so let me know soon what you think is a more concise title before too many more posts.
I did add some generic “tags” below for easier searches in the future. Anyone can add more “tags”at anytime that will help members in the future find this in-depth thread on precision tuning a Rotax carb equipped engine.

thank you sir! If we get some input as to better titles then I’m all for a change as long as it doesn’t inundate you with more crap to do (as if you’re not busy enough).

Nick

[bumsteer]

How about "Precision Tuning the Rotax 912ULS"?

Rick

[AvDES LLC]

Precision tuning the Rotax 912 UL/ULS - now that I like.

also currently building a set of intakes to bring these carbs closer to centerline while still allowing us to use the KF engine mount - nice thing as these intakes will already include the 1” crossover port.

mild stew for prototypes, fingers crossed this works and I’ll be producing a set out of 304SS for Bane.

[AvDES LLC]

Well yesterday was a success - the plane spent a few hours in the sky and I was able to monitor AFR throughout the flight.

During cruise flight at 5400 rpm I’m getting 12.8:1 on the head. This is beyond exciting. Now there is still a bit of tuning to be done at the 4800 rpm range as the plane reads an AFR of 14.1:1 - I’ll be ordering a few new needle jets to see what I can make happen there. Lastly idle is a bit on the rush side reading about 11.8 at 2200 rpm but it’s dead in 12.8 at 1750 rpm. I’ll be playing with the idle jets a bit as well to see what I can do with this due to the fact I’ve got the idle mixture screws set at only 1/4 turn out from fully seated.

overall the plane performed flawlessly and had zero issues with the maiden voyage. Heck yes back in the sky!

30331

[Slyfox]

let me ask you this, have you done anything with the floats. Seems the actual floats are junk

[AvDES LLC]

I have. I ordered the blue floats that are supposedly better than the stock floats as they are often mentioned to not deteriorate and absorb fuel over time. I did also end up having to set the fuel height in the bowl numerous times and current measurements have the float armature measuring 13.5mm from the carb edge as opposed to the 10.5mm constantly mentioned.

[Slyfox]

I've been having a lot of trouble with my carbs, flooding out while sitting without the engine running, running rough at cruise. I ordered the blue ones also, will be here Monday.

[AvDES LLC]

What do you have the float armature set to? Try 13.5 mm and make sure to do a full throttle grind run up before taking off. Run up for well past a climb out duration to make sure the carbs are refilling quickly enough after the adjustments

[Delta Whisky]

Congrats on getting back in the air and the AF ratios you have been able to set up. As soon as my new muffler is received I hope to do the same. Because I plan to follow in your foot steps, I do have to ask a question that is based more on my lack of knowledge about Rotax engines than anything else. Assuming that both of your reported AFRs are at cruise RPMs, why are you not pleased with 14.1:1 at 4800 rpm? You are still a tad rich of stoichiometric and (I'm assuming) not pulling so much power that additional temperature or timing control is needed. Thanks.

[Slyfox]

What do you have the float armature set to? Try 13.5 mm and make sure to do a full throttle grind run up before taking off. Run up for well past a climb out duration to make sure the carbs are refilling quickly enough after the adjustments
right now at stock. I've had so much trouble with these floats the last few months, now they are getting real bad, grounded until the floats come in. This thing will sit there not running and start leaking out the right carb, turn on the facet pump and starts pouring out. If I let it set a day with the fuel off and pull the bowl it will be about out of fuel in the bowl. I think it's soaking up into the floats. The other day I took the floats out of the left carb and put them in the right, took a set of floats that I had from years ago and put them in the left. I turned the fuel on and the facet pump, made sure the fuel was up all the way in the bowls. let it set for a couple days and check that right carb, fuel still at the proper level. stinkin' floats bad for sure there. now to set and wait for the blue ones. yes I'll set it to the 13.5mm thanks.

[AvDES LLC]

Congrats on getting back in the air and the AF ratios you have been able to set up. As soon as my new muffler is received I hope to do the same. Because I plan to follow in your foot steps, I do have to ask a question that is based more on my lack of knowledge about Rotax engines than anything else. Assuming that both of your reported AFRs are at cruise RPMs, why are you not pleased with 14.1:1 at 4800 rpm? You are still a tad rich of stoichiometric and (I'm assuming) not pulling so much power that additional temperature or timing control is needed. Thanks.

thank you sir - its been a hell of a time and im glad the big fight is behind me and the rest is just fine tuning. Now in regards to the 14.1:1 ratio - that is actually showing us that we are in a lean condition, which is why I am interested in tuning that out a bit further. If I were sitting in a slightly rich scenario (12.8 or below) then I wouldnt pay much mind to it as we rarely spend alot of time at the 4800 RPM range. My reasoning behind stating the fine tuning is worth the time is that lean conditions tend to destroy things and can result in some pretty bad things happening inside of the engine. That said, ill be spending a bit of time with this needle jet circuit to try and get these final lean conditions sorted out. I HIGHLY recommend welding in that AFR bung and spending a bit of time in the plane with all settings as they were before you did the AFR meter install and see where your engine was at according to those settings - you may be surprised.


Slyfox, While youre in those carbs, you may want to consider replacing the float needles as well. And while youre in there replacing those float needles, it may be good to polish the needle seats in the carbs themselves. You can do this with a hand drill, a few q-tips, and some metal polish - there are tons of videos on youtube that explain how to do this but lemme know if you cant find them and ill gladly do a small write up for ya.

[jrevens]

What kind of fuel are all you guys using… 100LL, 91 oct. ethanol-free mogas, 10% gasohol, or? Adding Sta-Bil to the fuel?

[Slyfox]

Slyfox, While youre in those carbs, you may want to consider replacing the float needles as well. And while youre in there replacing those float needles, it may be good to polish the needle seats in the carbs themselves. You can do this with a hand drill, a few q-tips, and some metal polish - there are tons of videos on youtube that explain how to do this but lemme know if you cant find them and ill gladly do a small write up for ya.[/QUOTE]


yes I did all that already. the floats are the last thing, should have been the first. lol

[PapuaPilot]

Shouldn't this discussion be in the "Engines" section? :confused:

[AvDES LLC]

probably. This thread didn’t start with this title or intention but evolved into a 912 tuning thread rather quickly. Maybe a mod can make that happen when they’ve got a free moment.

[desertdave]

Shouldn't this discussion be in the "Engines" section? :confused:


probably. This thread didn’t start with this title or intention but evolved into a 912 tuning thread rather quickly. Maybe a mod can make that happen when they’ve got a free moment.

Boom!.......... Moved to the engine section.

[Delta Whisky]

What kind of fuel are all you guys using… 100LL, 91 oct. ethanol-free mogas, 10% gasohol, or? Adding Sta-Bil to the fuel?

100LL with Decalin.

[AvDES LLC]

Boom!.......... Moved to the engine section.

Thank you much!

As far as fuel - ethanol free only - i avoid 100LL like the plague whenever possible - not good for the human body, environment, or the rotax.

[Flybyjim]

Swift 94 for the last 300 hours

[Rodney]

Appreciate your sharing your methodology and procedures. I have never felt my 912 was "tuned" properly either and have ordered an air/fuel gage to put in the muffler and
try your ideas.

One thought though - as you said you live in Reno at about 5000 ft. You have tuned your engine for that altitude. I live in Oklahoma at 500 ft. I'm curious to see if I end up with the same needle settings ( #2 slot) and close to the idle screw adjustments you have. My gut says your engine would be too lean down at 500 feet. Have you given any thoughts as to what you might need to adjust if you came down off the mountain? This should be a fun exercise.

Thanks
Rodney
N357RW

[AvDES LLC]

please keep us informed on your process and what you end up with. I have sun free made a few more changes and as such need to make a final update to the forum with the end settings. I know for a fact my final main jet is a 160 and needle clip went back to position 3 after that main jet went in. For some reason it ended up ridiculously fat in the mid range after that main jet upsize. Sadly when I pull back to 4800 ish im back in a lean condition, but all other settings im either slightly rich or dead on.

as for the adjustments when I come out of the mountains and back down to normal…. Nooo idea haha. I’ve permanently installed the ARF in my plane and will be tinkering as I play at different airport elevations. Yay self proclaimed test pilot :)

[AvDES LLC]

Weel have another update for the day - managed to kill yet another Innovate AFR gauge - thats three in just as many weeks. Soooo..... pulled the gauge and the sensor yet again, returned it for an AEM unit, and reinstalled. I will say that the inly change made to the carbs today was tinkering with float height a bit more and just before the gauge died i was able to confirm that I have drastically reduced the lean condition that was present before with just a small bump in the float bowl height.

Ive recorded alot of this and will be piecing together another video throughout this coming week to summarize what all was done and how the engine is now performing as a result of all of these changes.

thank you all for participating in this and looking forward to getting some data from those of you that have informed me youre going to go down this tuning rabbit hole as well.

[Delta Whisky]

Nicholas - I'm going to order the AFR unit this evening and that got me to thinking - is there a reason that you didn't install the sensor in the exhaust stinger? Thanks, Darrel

[AvDES LLC]

Good to hear you’re taking the plunge! I’m actually working on a split muffler right now to install dual sensors to allow for tuning each carb independently.

in regards to your question, the answer is a yes. They don’t recommend installing these sensors within 18 inches of the exhaust opening due to mixing of outside air and the likely hood of skewed results is high. I’d aim for dead center on that can of running a single sensor system. I may be considering using this split muffler I’m finishing up tomorrow as a tuning system for people that are interested in having tuning done, but in the long run don’t want to permanently install the sensors and gauges into their planes due to weight…. Stay tuned! Looking forward to hearing your results!

[AvDES LLC]

Boom! Tomorrow we see how each bank runs independent of each other.


3043430435

[Delta Whisky]

Inquiring minds want to know - - - - - - - -

BTW - my muffler and AFS are both inbound!

[AvDES LLC]

Well temp installed it and had to make a few final adjustments. Tomorrow should be the initial fire up with dual gauges and the split exhaust. I’ll make sure to record and post.

[bbs428]

www.summitracing.com/parts/avm-30-4110 (http://www.summitracing.com/parts/avm-30-4110)

www.summitracing.com/parts/avm-30-0300?rrec=true (http://www.summitracing.com/parts/avm-30-0300?rrec=true)

Would these kits all I need to monitor my a/f ratio?
Appreciate all your work here. Gives me hope that my carb's can be tuned to a higher degree of efficiency.

[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! :D

[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.

[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.

[Delta Whisky]

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. :cool:

30584

[AvDES LLC]

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

[AvDES LLC]

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:

[Delta Whisky]

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.

[AvDES LLC]

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?

[Delta Whisky]

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

[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.

[trond]

needle is already in richest position

[AvDES LLC]

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

with the lean condition present at the higher RPM range I would bet money on your mains and not your needle jet. As stated my Delta, ensure the needle is at richest position and if so, I’d step the mains up first. Remember that when you tune carbs, you typically tune at wide open throttle first and then DOWN the rpm range, not up. With your lean considering being in the 4K and up range, that’s where you’re carbs are relying on the main jet as it’s metering jet, not the needle. Once you open up that main, you’ll likey have to bring clip position back down so you’re sitting in that sweet spot during mid range. Just my .02.

[trond]

here is a good read on Bing tuning. Basically a CV carburettor (constant velocity) does not behave same as a normal slide carburettor. It is the load/manifold pressure that dictates slide&needle position, and not the RPM or throttle setting (obviously at WOT there will be high load too unless diving).
The fact that I get 12.5-13 at WOT makes me believe I have the correct main jets. I have already richened up the needles to the btm position
If I go to larger jets I will be running too rich at full throttle/load settings. I think

At cruise it is the needle jet that is most important, not the mains

But I take your advice and have added larger mains to the order from Bing just in case I should be wrong ! Hopefully they will arrive in a week or two and I will post an update



http://jabiru.no/content/documents/motor/Tuning_the_Bing_Carburetor_on_Jabiru_Aircraft_Engi nes.pdf

Climb out: Since the amount of fuel delivered to the engine at throttle settings of ¾
open or more are controlled by the main jet, we make a change in the main jet to come
closer to our target of 1225. If climb our EGT’s are higher than target the mixture is too
lean and a larger main jet is indicated. If EGT’s are lower than target the mixture is a bit
rich and a smaller main jet should be tried. Remove the main jet to see what size is in
the engine. Main jet sizes are a three digit number and most likely 255 for a 3300 and
245 for a 2200. Adjust up or down by 5: ie 255 to a 250 or 250 to 255.
2. Cruise: At settings of ¾ throttle or below the limiting factor for fuel delivery is the
needle jet. Cruise flight should be at settings at or below ¾ throttle in most cases. The
target for the center of the range for cruise flight throttle settings is 1325 F. Based on
your EGT observations in cruise flight install a larger needle jet if temps are above the
target or a smaller needle jet if temps are below target. Needle jet sizes are numbered
in x.xx format. You may find a 2.85 needle jet in your carburetor and will need to try
perhaps a 2.88 if temps are a little high or a 2.90 jet if temps are quite high. If cruise
temps are too low then a smaller needle jet is indicated

Note that the above numbers are for Jabiru with Bing carburettor

[AvDES LLC]

with the lean condition present at the higher RPM range I would bet money on your mains and not your needle jet. As stated my Delta, ensure the needle is at richest position and if so, I’d step the mains up first. Remember that when you tune carbs, you typically tune at wide open throttle first and then DOWN the rpm range, not up. With your lean considering being in the 4K and up range, that’s where you’re carbs are relying on the main jet as it’s metering jet, not the needle. Once you open up that main, you’ll likey have to bring clip position back down so you’re sitting in that sweet spot during mid range. Just my .02.

oh crap I read that wrong - you are 100% correct. I thought I read that you were getting lean at 4K and staying lean throughout the rest of the range. Re-reading it, you’re dead on the money at the upper end and as such my entire previous statement went in the crapper. Shouldn’t read posts while sitting in the shop trying to focus apparently.

[trond]

I have had an issue with lean mixture and vibration at cruise and ordered some larger needle jets from Bing. Today I tested with 1 step larger needle jets, up from stock 2.72 to 2.74. I did not see much difference on the AFR in cruise, if any. Mixture was perfect 13-13.5 from 2000 up, and in climb after takeoff 12.5-13 which is good for power. But when I level off and throttle back at 5100-5400 I see AFR around 15.5. It felt like engine was running smoother though.



So I landed and changed up one more step, to 2.76. Minimal difference. AFR at cruise 15.2-15.5. If I pitch nose up AFR drops to 14.5. Needles already at richest position. Spark plugs look nice and grey/tan with clean insulator down to the root.






One thing I have noticed is at lean misfiring the AFR shoots right up, with the unburned gasoline in the exhaust. So it is possible to get fooled by false readings. And maybe what I read now with less misfiring are more solid numbers ?


Don’t know how to proceed. Maybe leave as is and remove AFR meter. Maybe order even larger needle jets. I only increased by 2 points which may be very little ? Oil in the KN filters ?

In my motorcycle days one trick to get the CV carburettor to work on the KTM 640 without a proper airbox was to cut shorter the spring inside the throttle slide....




11.5 AFR - Best Rich Torque at Wide Open Throttle (WOT)
12.2 AFR - Safe Best Power at Wide Open Throttle (WOT)
13.3 AFR - Lean Best Torque
14.6 AFR - Stoichimometirc Air/Fuel Ratio Value (Stoich)
15.5 AFR - Lean Cruise
16.5 AFR - Usual Best Economy
18 AFR - Carbureted Lean Burn Limit

[AvDES LLC]

when performing full power run up on the ground what are you seeing for AFR? Have you ran a wire through your main jets yet at any point during disassembly for the needle jet exchanges?

[trond]

I see 12.5 -13 on full power static. Same as on WOT climb. It is only when load is reduced it goes lean. I have decided that only 2 steps up was too fine steps and will order more needle jets. Should be here in a month... meanwhile I can at least fly as it runs better already. Thanks for inspiring me to install the AFR. I use them on bikes and cares but for some reason I did not occur to me on my own that I should have one in the plane. I really would have liked a split exhaust to do it properly but L & R spark plugs read similar so I will use the AFR as an indicator only (as it should be) to see where changes happen and how large the changes are. And double check with plug readings

Main jets are new and very clean. Inspected them today even

[AvDES LLC]

Hmmm. Those static numbers seem pretty good. Have you verified your float level? You’re be surprised just how much the level of those floats actually impacts AFR.

if you’re every wanting to tune with a split exhaust we offer the one I’ve developed for rent to other 9xx series engine owners. Please drop me a PM if you’d like to discuss independently tuning each bank.

[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? :confused:

[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? :confused:

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? :confused:

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.

[trond]

But I believe it IS broken. With stock jetting my 912UL with KN-filter pods run so lean at cruise that I fear burned valves and gearbox damage from the vibration I can feel. Only at cruise
I am trying to richen it up at cruise is all. I have heard others with same set-up (912 UL and no airbox) say 'thats the way they run' but I think there is a lean-condition caused by the removal of Rotax airbox. Without an AFR meter I would be less aware
Google is full of posts by motorcycle owners struggling with CV carbs running too lean after removing factory airbox. The stock carb set up recommended by Rotax was made for airbox applications I believe

But you are right; some of us have a tendency to obsess. Having an AFR meter makes it hard not too when the needle is at 15.5




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? :confused:

[AvDES LLC]

I look a thte AFR meter as yet another means to analyze the engine actively and keep an eye on the health of the engine. Im a big proponent of safety and having another tool at my disposal that is mounted in my panel that lets me view dynamic information about the status of my engine is a good thing. Some call it obsessing, I call that safe piloting.

[jrevens]

… The stock carb set up recommended by Rotax was made for airbox applications I believe…

I could be wrong, but I don’t think that’s true. I believe the airbox was designed and became available from Rotax later.

[AvDES LLC]

I think we all need to recall that these carbs were actually designed for use on BMWW motorcycles, not original Rotax Equipment. With that in mind, how did the BMW bikes do it?

[jiott]

AvDES, after all your experimenting to this point I believe you are claiming a 900 rpm improvement among some other things. I have been following this but have become somewhat confused about what actual changes have been made to get you to your best possible overall performance at this present time. For my benefit, could you just simply summarize all the carburetor changes and adjustments you have made compared to factory stock. Thanks.

I do need to keep in mind you are working with a 912ul, not a uls like I am running.

[AvDES LLC]

Hey Jim, thanks for following along. Honestly the best way to put together exactly what I did and the improvements that resulted would be to watch the videos ive attached to this thread in chronological order. The adjustments I can recall from memory right now were new larger main jets, needle clip position, fuel bowl fuel height level, etc. If I have a moment this weekend I can try to re-watch those as well and provide a little write up - time allowing of course.

[trond]

you will find the Bing carbs work well on BMW motorcycles as long as airbox is retained. With filter pods not so much. As with most engines that run CV carbs of any brand. I have had the same issue with Mikuni CV carbs on Kawasaki and KTMs; very hard to get them to run properly. In fact it is almost always a problem to get any CV carb equipped engine to run as well when airbox is replaced with pods and the common problem is they run too lean at 3/4 and up. With a CV carb it does not necessarily help to increase main jet size as the problem is in how the slide operates

I tried today to reduce needle jet back from 2.80 (that gave overly rich condition at 3000 rpm AFR 10.5) to now 2.76 and now see 11 at 3000. Still rich but better. I also cut off two coils of the spring to see if I can get more fuel at cruise. Hopefully there is a weather window the next days to test. If too lean at cruise still I will raise needle from next richest to richest position. Don't know how to fix this really and just testing ideas.


I think we all need to recall that these carbs were actually designed for use on BMWW motorcycles, not original Rotax Equipment. With that in mind, how did the BMW bikes do it?

[AvDES LLC]

sadly perfection at all rpm is unlikely due to the nature of how a Cv carb works. If you’re trying to hit 12.8 - 13.1 at all ranges of throttle, the only way you’ll ever make those numbers is a F.I. System that can modulate injector timing and pulse duration in an attempt to actively even out the stoichiometric ratio being read by the computer.

I’ve been tuning bikes and carbureted off road race vehicles in this manner for years and have always had at least one portion of the rpm range that doesn’t play the way you’d like it to - in those instances, we would tune the fuel system to the engine such that the area of adverse values remains in a region of the rpm band that is not relied on very often - the trucks and buggies we did were mid range fat just like these Rotax engines are showing. With us keeping those trucks spun up at the top end of the range the only real regions of interest were the final 1000 rpm and the idle / low range circuit. As long as we kept it fat in the mid range, we had great cylinder cooling and ZERO on-site engine work need during races.

Sadly my bet is that you can tinker with this range all you want and upon you getting your mid range locked in, you’ll see something you don’t like at the top or bottom of your rpm band. You’ve officially hit your point of diminishing returns. Go F.I. If you want perfection at every point of your rpm range, or aim far for the mods, button the plane up, and go fly it.

[trond]

AFR range where engine runs fine is actually all the way from 10.8 to 15.2. My only aim is to not run as lean as 15.2 at cruise where the engine spends most of the time, and also 12-13 on WOT take off and climb out. Once that has been achieved I consider removing the AFR meter, to help me relax. If EFI was less expensive I would convert. I have looked at a friends plane with Edge EFI and it is a thing of beauty both how smooth it is and the way it is designed and built

[AvDES LLC]

runs fine and most efficiently are two very different things. The engine shouldn’t bog when seeing the numbers you’re providing, but you’re also not pulling peak power at any rpm in which your ratio is outside of 12.8 to 13.1 ( I believe the scientifically proven upper threshold is actually more around 14 but I aim for that 13.1 for consistency).

those FI systems are very nice, but I’m with you - I cannot justify that level of expense for my little 80hp sky dirt bike. Maybe if there was a way to make some of that money back, but since I’m no competitive racer or silver spoon child, I’ll stick with the carbs for now haha

[trond]

I am at exactly 12.8 at WOT so that is good. This is important as you say to get power

Cruise now at 15 and it runs fine there too. Lean stuttering now gone at cruise which was priority. Much better than it was. I only cut off two coils from the springs but it was an improvement. Supports the vacuum theory. Might try cutting off one more coil later or not, for now I am happy and plane is operative.




runs fine and most efficiently are two very different things. The engine shouldn’t bog when seeing the numbers you’re providing, but you’re also not pulling peak power at any rpm in which your ratio is outside of 12.8 to 13.1 ( I believe the scientifically proven upper threshold is actually more around 14 but I aim for that 13.1 for consistency).

those FI systems are very nice, but I’m with you - I cannot justify that level of expense for my little 80hp sky dirt bike. Maybe if there was a way to make some of that money back, but since I’m no competitive racer or silver spoon child, I’ll stick with the carbs for now haha

[jiott]

I wonder what the AFR numbers are thru the rpm range for a 912 with Edge EFI system, or for that matter what they are for a 912is? Would be interesting to put an AFR meter on those engines-maybe someone already has?

[AvDES LLC]

I wonder what the AFR numbers are thru the rpm range for a 912 with Edge EFI system, or for that matter what they are for a 912is? Would be interesting to put an AFR meter on those engines-maybe someone already has?

apart of those systems REQUIRES that an AFR meter be in place. The computer “sniffs” with the O2 sensor and adjusts the parameters as far as pulse peak width and duration in an effort to hit the AFR the system is set to operate at. I cannot make any statements for edge, however I can make statements as far as BDTurnkey’s Yamaha conversion EFI controller and I know for 100% certainty that system is set to operate at exactly 12.8 to 13.1.

[trond]

31583

This an from the dyno. 912 ULS with big bore kit and Edge EFI


I wonder what the AFR numbers are thru the rpm range for a 912 with Edge EFI system, or for that matter what they are for a 912is? Would be interesting to put an AFR meter on those engines-maybe someone already has?

[AvDES LLC]

Thank you for posting that! Looks like their range is 12.2 to 13.4 at the extremes. Depending on sampling rate of the system it could also be that the data samples were pulled at major peaks / valleys and those points were attempting to be corrected by the system at that instant. Either way this looks to align with the numbers I remember from many many moons ago when I was a kiddo being taught this stuff by Grandpa and Uncle T.

[trond]

it certainly looks great. Awesome system. Obviously this is at static and does not show cruise values that are bound to be leaner

[AvDES LLC]

Why would you say that cruise values are bound to be leaner? Interested in your thought process on this one.

[AvDES LLC]

Ah. Thanks for the response. I’m aware of the 3D maps, I thought you had some first hand insight / knowledge into the Edge system that the rest of the world isn’t privy to about their maps or settings. I see now it’s just based off of the standard tuning assumptions.

[Shadowrider]

I know many that are running the edge efi and love it on their 912ULS. If it was in the budget the first thing I would do to the 912uls is put EFI on it. The guys that have done it say how much smoother the engine runs. Starts like a modern engine. Freezing cold, burning it hot, they don't care. You can richen up the lean midrange and lean out the rich ends. Keep in mind the dyno chart you listed is for a turbo 912uls (edge915) so the AFR are a little different. The 912is and 915is are trying to acheive peak fuel efficiency so rotax runs them very lean imo. Also turbo engines needs richer AFR's to help stay away from det.

[AvDES LLC]

100% agreed that a FI system will be leaps and bounds above tinkering with carbs to get them into "best range". Another thing I find very interesting is the differences in "mission" that people have with these aircraft - some are worried about peak fuel efficiency which is not peak power whereas I am one of the individuals that is tuning for peak power and not really taking fuel efficiency into consideration. Depending on the goal of the plane - short back country strips with rising terrain and mountains on either side is the kinda flying i play with and in these instances I want as much power as possible at that prop, not best fuel efficiency. If I had the desire to go FI for the consistency and the ability to tinker with the maps, id still likely aim that system toward the 12.8 - 13.1 AFR that I continue to mention - that would give the best of both worlds from the smooth engine perspective while still keeping that engine in a fueling range that provides maximum cylinder cooling and power simultaneously. Just my .02

[trond]

I have solved my carburettion issues by installing LAD Aero EFI. Engine is very happy and purrs at any RPM. It idles nicely at 1500 RPM. It was a easy and clean intall and I found user support to be excellent. I went with lambda control and automatic switchover to back up fuel pump. Also changed to a Shindengen RR and warning LED and it seems to work well. Have 25 hours on the install now and could not be more happy

[mike67]

Hi guys


there is now a distributor in Canada for the LAD aero injector kit

[Dusty]

I've recently been down the tuning rabbit hole with a 912 ul zipper. I hope my findings may be of some help to someone.
My starting point is a late 300 hrs ul. It ran ok at all aspects of flight. My flying is mostly 1000 to 4000 ft with occasionally 10.000 ft
Fuel is 95 octane .float levels are 12.5 mm from the fuel bowl edge.
I have always experienced high egt at wot. I installed a 1 inch balance tube in preperation for a zipper install ,the idle was vastly better .
The 104 hp high torque kit was fitted . The exhaust valves showed signs of heat stress. 915 valves were fitted to cnc machined heads.
Wot egt was still high and an exhaust gas test confirmed 15.3 so we installed 165 mains which helped a bit.(158 std)
On a hunch I believed the needles were limiting the fuel at wot.
I removed the main jets completly and did a run up. The egt went low at wot but all else stayed the same, bingo!
The needles were reprofiled in the lower 8 mm similar to the Jabari carb.
We re checked the ratios (#30 idle jets were also fitted as the idle screws had to be nearly all in for smooth idling at less than 1700 rpm
Although not technically perfect I am more than happy with egt's and smoothness at all throttle settings.
I will post the gas results when I find the safe place I put them.
These results may not work on a uls as the fuel delivery is quite different to the ul

[AvDES LLC]

Great input Dusty! If youre at all wanting to hone in on how the engine is performing from a fueling standpoint throughout the RPM range, an AEM AFR meter is a cheap means to do so. I am not at all surprised that you ended up stepping up the main jet as that was one of the issues I had at 6000 ft where I currently reside. Good addition to the conversation!

In regard to the mention of EFI - I definitely agree that EFI is a way to go if youd like to have an almost plug and play system. For a majority of people flying behind a ul or older ULS, it doesnt make sense from a cost standpoint to put a few grand into a fuel delivery system. that my .02 and im POSITIVE there will be those that do not agree with that sentiment.