Please be cautious about adjusting right thrust.

The way I determine the proper amount is to put the plane into a vertical line, and then cut power to less than needed to maintain the climb. When the plane nears zero airspeed, it should continue straight and not drop off to either side.

At flying speed the rudder controls yaw, but below that engine thrust angle takes over. If in your testing the plane was flying and you had control, maybe the pull to the left was something else other than precession/P factor? I know when I push the throttle open I have a tendency to also move the rudder.

Tom Fawcett