The problem is not your power setup. It is with the plane. Funtana 40's rudder has insufficient yaw authority. You pretty much have to get a full blast and full rudder once the plane yaws off. I have tried many props on my FUntana 40 w/ Saito 91: APC 15x4W, 14x6, and 13x8. None seems to help the weak yaw authority. I even supersized the rudder, which only increase high speed sensitivity, but does little for low speed. I have concluded that the fuselage shape is very unhealthy for the rudder.