912 ULS Rough Running Low EGT - Solved!

You could have something in one of the carb orifices. I think I would pull it again and flush out all orifices with carb cleaner in a spray can followed by some high pressure air. Also check the float height armature. 10.5mm from the carb edge to the top of the brass float armature and like Xing suggested make sure you have no leaks around the rubber flange and that the carb vent tube is the same length and same basic location on both carbs. i.e. under the carb bowl bale

Roger and Xing,

The only thing affecting running and egt at full power-- in flight but not on the ground-- seems to me to be the diaphragm and/or needle.  Would a leak in/at the diaphragm cause the right side to go way rich or over lean enough to run rough with low egt?  I guess the carb has to come off again and get a real going over.

I thought this out, I believe:  A leak in the diaphragm would cause the needle to drop, restricting the fuel and causing a very lean condition.  We have never observed any sooting of the plugs, which is consistent with very lean operation, which must be so lean as to be far over peak, dropping the egt.  Carb is coming off.

Les,

The fact that your problem is only in flight makes me wonder how your carburetor vent lines are routed.  Are they short lines tucked in out of the slipstream as Rotax recommends?  Just a thought, take a look at this video: 

https://www.rotax-owner.com/en/videos-topmenu/rotax-minute/454-carb-vent-lines

Thank you Jeff.  I had earlier seen another reference to the vent lines.  Mine are short and tucked in as Rotax and Van's recommend.  I did find another tip and tried it today:  I flew the plane and used the choke control to enrich the mixture when the EGT's were mismatched and the engine was rough.

Adding fuel lowered the already low right side EGT immedieately, indicating that it was already RICH.  The higher left side EGT dropped with more enriching.  

The only cause of enriching in flight not on the ground which I can imagine is a plugged vent orifice.  This would keep the float bowl pressure from decreasing aloft and push more fuel through the main jet than should be, causing a rich mixture.  Other plugs would create a lean condition, it would seem.

Any other ideas?