912 ULS over 200°F EGT difference

Plugs can be sooty if run at idle too long. Those are both sooty and look as if they were run too much at idle before you checked them. Run the engine up around 3500 - 4K rpms for 10 minutes or go fly for 15 minutes at normal cruise rpms. When you land turn the engine off and don't do any long low rpm taxi back to the hangar. The low rpm soot will burn off at higher rpms. Then look at your plugs to get a better feel for the burn.

As for a 200F EGT temp it is significant in certain circumstances. How close are the EGT's at cruise rpm? have the carbs been worked on lately? How did you do your pneumatic sync at at what rpm? Just a mechanical sync by itself isn't enough.

How about your mag check numbers? Are they reasonably close? 

If you live and fly out of a high elevation then the carbs can be leaned a bit, but unless you are armed with the correct information that would suggest that needs to be done leave it alone.

Thank you for your quick reply.

I will follow your advice and check the plugs again when I'm flying the next time. Probably tomorrow.

At cruise speed - around 4500rpm - the EGT's differ by about 220F.
The carbs have not been worked on lately but the problem has always been there (since 300h). Unfortunately I don't have a rotax expert nearby. My mechanic sees no problem but for me the difference doesn't seem right.
I did the pneumatic sync at 2600rpm and additionally checked at 3300rpm.
My mag check numbers are fine - rpm drops by 200 rpm.

So your advice would be to leave it alone if the plugs are good after the next check?
If the plugs are stil very sooty, is there a way to lean the 912 at cruise rpm? In the Rotax documentation they only speak about "idle adjustment".

Your cruise rpm in the mid 4500 rpm area is too low and may be adding to your issue. Fly it around 5100-5300 rpm for cruise. This may depend a little on your prop. 200 rpm drop for a mag check is fairly high. Are "A" & "B" ignitions fairly close in rpm drop?

When you synced at 2600 were they still in sync at 3500+?

My personal preference is to sync carbs at the higher rpm like 3300-3500 and then down at idle. I have found too many times that when I just sync at 2500 like the book says the needles on my gauges start to split again as rpm is increased. I seem to have a more solid and consistent sync all the way up when I do it at 3300 vs 2500.

Again this is personal preference.

I already suspected that my prop may not be correctly adjustet for the plane. If I am flying 5300rpm horizontally I am already at full throttle.
My prop is fixed, so no variable pitch. Do you know where the max rpm sould be? Maybe I am able to fix the pitch this weekend, a good friend of mine has everything needed.

When I am out flying tomorrow I'll note down all the exact details, as well as the exact rpm drops during mag check. I think the rpm drop from A and B is eqal but honestly, I am not sure.

Thank you very much for your help so far, I will update you tomorrow with exact figures.

If you want a well balanced performing prop for climb, cruise, fuel economy and engine temps then shoot for 5600-5650 WOT rpm. Then cruise around 5100 - 5300 in normal cruise. Set this max rpm at your average flight altitude. We all know sometimes we’re lower or higher so we’re looking for a balance. If you have a need for better climb over cruise then use 5700-5750 ( it can be up to 5800)