I've seen something similar happen to another UCD.

The problem was too much pressure and the high end needle was turned in to compensate. This gave a very sensitive mixture, and actually caused a deadstick that led to a crash destroying the plane. His needle valve was only open about 5/8 turn from fully closed.

Another flyer has that engine now and set it up at about 1 1/2 turns open, then back down the pressure until it runs right. No more problems!!

Just something to check for. Hope this helps.