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

[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.

[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.