While I do love biplanes, they are twice as much "fun" to rig, when it comes to setting incidences and such. If it turns that hard to the right, you either have a warp in one or both wings, or they are mounted in such a way that tightening them down is pulling something out of whack. Look at the plane, set level on a table in good light, from the rear, looking at how the trailing edges appear, relative to each other, and from one side of the wing(s) to the other. You ought to be able to see a warp that bad. I presume the rudder is straight, and the turn you refer to is a roll, not a yaw?
On a somewhat unrelated note, there are some other threads that address a commonly agreed on design flaw in the Sterling Stearman concerning the incidence setup in the plans. Most biplanes seem to like the bottom wing set at or near zero and top wing -1 or -2 degrees incidence relative to the bottom. Might be worth hunting them down and reading.

Rick