Just give it some more breakin time.

I had one on my Ultra Stick. After four breakin tanks it was giving me nice power at the top, but I had to run the idle moderately high. It also had a tendency to load up and sputter/die when run at low throttle settings - which it did one windy day while I was way upwind. I misjudged my landing approach, and broke my wing/fuselage/landing gear/motor mount. Oops!

Didn't hurt the engine, though. I put four more breakin tanks through it on the stand, and leaned the idle needle. It seems fine.

My replacement Ultra Stick has arrived and is in the building queue. As soon as I finish fixing the crappy wing aileron mounts in my buddy's GiantScalePlanes P-47 (unpainted white plastic ABS covers, screwed into 1/16" balsa sheeting. Arghghghghg!) and repair my Durastik (battle-scarred combat plane), it will fly again.