The rudder is 14 inches tall with a chord of 3.75 at the top and 6.75 at the bottom. It also has a aero counterbalance of roughly 2x3 inches or so. Also, the RPM estimate was conservative. I usually tach the engine at 9000 full throttle on the ground. I don't know how much it unloads in the air. I am usually at 3/4 throttle or so, but with these 2 strokes the throttle is not at all linear, so who knows what rpm it is really running. It seems like the blowback occurs a second or so into rudder application. Maybe it is an issue with sustained current heating up a component or a battery going flat. I am running 1800 mah. Nicads with 5 cells, so I doubt the battery is at issue. I guess you can't count it out though!
Furthermore, the Servo rotation is much less than 140 degrees for 40 degrees of surface deflection. It is closer to 70 degrees.