Balance according to the manual. Instead of adding weight try moving the battery/receiver as far aft as possible. On my US 60 I mounted the rudder and elevator servos in the tail and mounted a 1400mAh battery a few inches aft of the servo tray to acheive balance without adding any weight, and thats with an O.S. 91 Surpass, which I would suspect is a few ounces heavier than your 61FX 2-stroke. Good luck! By the way, you're going to love this plane! Later.