Ed, the .91FX is a ringed engine.

In other threads, people have said they have good luck with the Perry Remote Needle Valve, but they are the ones seeing air bubbles between the needle valve and carburetor.

Try tightening the head bolts.

What I noticed on my 1.60FX when it was new was that it took nearly a gallon of running on the ground before it would run reliably in the air. One night I deadsticked it six times in a row. The next day I tethered it and ran three tanks through it gurgling rich before trying to fly it again.

It's a fact that ringed engines require more breakin than ABC types, and that breaking them in 4-stroking rich is the way to go.