Sounds like you changed the thrust angle. The prop (an airfoil in its self) thrust will cause one side to pull (lift) greater than the other, that is why the engine is offset to equal out the pull. The vert. stab or ailerons or wing may be warped causing too much deflection in the airfoil. This deflection, plus the thrust, would cause the plane to go right and changing the thrust angle to 0 or - (left) balanced out the difference. You should check to see if the vertical stab and rudder are strait, that there is no difference between the airlerons when at neutral, and that there isn't any warp in the wing. If any one of these problems correct themselves, and they could due to temperature or flying conditions, then you will have a plane that is very hard, if not impossible, to control.
Scott