Sean,
I have almost the same problem as you do, only it's with a MVVS .21. I installed mine in a Wonder for some high speed fun, it ran great on the stand and on the plane untill I tried to fly it. Boy, what a dog! I tried everything. I tuned and tuned untill I got so mad I took it off and installed another engine. Plane flew great after that. I installed back on the test stand and it runs great! I still can't find the problem with this engine. I called Morris and he said that it will "unload" in the air and for me to try running it "real" rich. Tried it...didn't change a thing! I put the other engine back on and just gave up. My MVVS is now a shelf queen collecting dust. This makes me real mad too, because I'm a mechanic by trade and have been fixing engines since before there was dirt! This one kicked my butt good. I tried all the same stuff that everybody has said, low/no nitro, cold/hot plugs, new tank and lines, resealed the carb and backplate, rebuilt the carb, tried everything I could think of and everything anybody else could think of with no help. My mommy didn't raise no fool, I know when to quit! I hope you have better luck than I did.
Suptter
Danny Mercer
Rome, Ga.

Hey...Look....we have the same last name...same engine brand....Maybe it's a bad Omen