fuel/air mixture

Why did they put it in their  Manual is still the question?  Cars , bikes ,snowmobiles etc run best at certain temperatures and many have computers that maintain those temps. SO why are these well designed engines any different?  I guess I can't get my head around it.

Too Be honest running these engines at 5500 all day which they are designed for  seems like more of a worry than EGT but that is another subject.

Thanks for all the great input

I agree with what everyone is saying but I was curious about "always setting idle adjustment to 1.5 turns" and "always setting needle height at position #3".  I remembered previously reading posts where owners have noticed different setting from the factory.  I looked at the heavy maintenance manual for the 912 and it does not state to automatically set the idle adjustment to 1.5 turns.  The manual says to note where it is set on dissasembly and then set it the same way.  In reference to the needle height the manual specifies position #3 except for the 914 which may have a different height for the left/right side.  In reading the manual it is pretty clear that the idle adjustment screw can be set differently from left/right.  In my experience on my engine, it has run much better when I used the EGT readings to fine tune the idle adjustment screw.  I agree that a lot of things affect the EGT.  But at any given time both carbs are experience the same altitude, humidity, etc.  In the end, I am satisfied my engine does not have a rough spot like many owners experience in the lower RPM's.  I realize we all want our engines to run good at "cruise" but I also like mine to not have a rough spot at the lower RPM's.

"I looked at the heavy maintenance manual for the 912 and it does not state to automatically set the idle adjustment to 1.5 turns."

Hi Bill,

First Merry Christmas. :) 

Okay,,,  Wrong manual. It's the Line Maint. Manual. Section 12-20-00 page 38

Remember all the things we do is to make our engine reliable and have a long life. Rotax has had 6 million run hours to to figure this out. I have friends with 2K -4K hours on their engines. This is what we are all shooting for. RELIABILITY.and our SAFETY. 

Unless the EGT's are way off you don't care what they are to a point so long as they are in the same ballpark. Max difference is usually less than 120F.  Many I see are between 40 - 90 rpm difference. If they are within 15 degrees then they are considered the same. Your EGT probes can easily be off too. You may be relaying on these to be overly accurate. They aren't as time accumulates.

Items to consider: Each carb may have a minor different air flow, you may not have all 4 EGT probes (many users just have two), front, rear and left vs right cylinders may have a minor different air flow (you can notice this when you idle too long and find some plugs clean and other are dry black soot)

We care more that the carbs are equal on each side and that the left and right side run equally at the same rpm, not the same EGT. It might be really close, but usually not completely equal plus this changes with rpm. (idle, mid-range and cruise rpm's) Otherwise one side will be trying to operate at one rpm while the other is trying to run at another and you'll never feel it. Trust the gauges and all the Rotax's millions of run hours, testing and research. If the carbs are synced properly then the EGT's will be within their proper range.

Trying to make things perfect or equal to what YOU think it should be could lead to other consequences down the road. It just cost one guy at my field $2800 to rebuild his gearbox because he damaged the dog gear hub from the vibration he couldn't feel.

Bill,
Yes, the 914 will have the needle factory set at either position 1 or 2 for the 1/3 cylinder carb, and position 2 for the 2/4 carb.  I was referring to the 912 when I stated position 3, and I should have said use the “factory settings”. However, in my opinion there would be no reason to change this from the factory setting unless the configuration of the intake air system was modified.  It would seem difficult to make a better (overall) calibration than Rotax does on their test bed. 

 As far as setting the idle screw to 1.5 turns out, that is stated in the synchronization procedure as part of making the basic settings before beginning the sync. As far as I can see, there is no further adjustment recommended.  I assume this is a setting that is known to work well in a wide altitude range.  To the best of my understanding, the idle circuit in these carburetors is not self-compensating for altitude in the same way that part throttle and full throttle operation are.  So any adjustment of the idle screw would be for a specific altitude, and may affect how the engine would perform when pulled to idle (say on approach) at an airport of a much different altitude.  

Don't compare the 914 setup with the 912ULS. They are different animals on the fuel system setup.