I am one of the seemingly few Rotax 912 pilots with a manual mixture control, and for the last few months I have been stumbling my way through figuring out how to use it. The Rotax uses carbs with a rubber diaphragm that auto control the mixture, to a point. Somebody figured out there was a fuel savings to be made with additional leaning at altitude, rediscovering the wheel, I know, and marketed the HacMan Leaner for us 912 guys. A hell of a deal for around $200.00. The after market device I have works in conjunction with the auto control. In practice, I use it like "you guys" (Lycoming/Continental pilots) use yours, sort of. Basically, if flying around at or below my 5140' field elevation (I live 1,000' above the valley floor), I don't mess with it. But once up higher and planning to stay there, like a 9K+ flight today to Wyoming, I lean until I get a stumble, then I richen it up "a bit". That's usually about 30 degree according to my EGT, it gets cooler, I usually end up around 1425. If I continue to climb, I'll go through that simple procedure again, each time the fuel flow meter clearly shows the dividend:.4 to .6 minimum LESS fuel flow per hour, substantial, percentage wise, considering we're talking 3.2-3.4 GPH instead of the unleaned 3.7-3.9 GPH.
I have no one flying the Rotax with one of these leaners that I have got together with and compared notes, I'm on my own and pretty much try and take what you guys do and extrapolate it for the Rotax. Apples to oranges though, like my CHT's today were 140 to 160 degrees! One thing I have found however, is if I get her all leaned out real good at 10K or higher, and then zone out and forget all about it and land at 4 or 5K, the engine will quit, right at short final but I had the runway made so no big deal and a great way to educate myself. Another time, after being leaned out just right for 10 to 11K, I landed a 9200' site without re adjusting things. On shut down I got one hell of a loud POP, like I had never gotten before, a backfire. I don't lean for takeoff, I just let the auto control get it close enough, it's just on the long high flights I really find it pays off.
My plugs look great, all the other temps are good, so I'll continue to operate it as is. However, I have not tried going leaner after that initial stumble, that's how you get to lean of peak right? Hope this doesn't confuse your issue Anne, but I feel I can chime in with the others and say that leaning really pays off, however and on whatever, you do it.