Precision tuning the Rotax 912 UL/ULS

[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?

[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