I do not have any specific knowledge of this plane, however something does not sound right. The plane needs to be perfect on balance or nose heavy a smidge in order to fly. The OS 40 La is a light engine so nose weight is in order, although I would think only a few ounces of lead would be needed. Is it possible that you are checking for the balance using the wrong measurments for the balance point on the wing. Typcally using the spar as a starting point is good. Good Luck, Dave