912ULS runs rough and limited to 3400 RPM

Sounds like carburetor issues. Suggest doing a mechanical balance and pneumatic balance.

This is very embarassing.  I'm not sure why I am sharing it.

Upon opening the carb on the right side, I found the metal plate and the four screws that hold down the base of the diaphragm loose; or, rather, NOT installed - just sitting in there, loose.  

I had taken both carbs home to work on them.  I reassembled both, but before finishing the right one, I suddenly relaized I was late for a Zoom meeting that I was leading.  So, I put the plate ands crews inside the carb, replaced the top and threaded the screws on just tight enough by hand to keep anything from falling out - I would do the final asembly at the hangar later that day.

Well several days went by without gettng back to the job, and when I did, I just installed both carbs.  Didn't remember the right one needed just a bit more work first.

Conclusion:  I have way too many distractions when I'm home with wife and dogs and cats, and the ringing of the phone with all the political solicitation calls. The hangar is quiet, more or less free of distractions (especially if I park around the corner and lock the door!).  

"This is very embarassing.  I'm not sure why I am sharing it."

Thanks for sharing. Valuable lesson.

Whew!  Glad to know there was an explanationn for what you were experiencing...  And even more glad that the lesson learned (for all of us) wasn't too painful!  

Will Rogers said "There are three kinds of people:  Those who learn from reading, those who learn from other people's mistakes, and those who have to pee on the electric fence themselves."  And as they say about landing with the gear up, "There's those who have, and those who will..."  We've all done screw-ups like this, and it helps to be reminded from time to time that NONE of us is infallible.  Check, and double check...

Thanks for posting... there are a number of things for which I would NOT want to be distracted while doing, and reassembling a carburetor would be very high on that list!