you can adjust the engine slightly for balance,but the cowl needs to be set the right distance from the prop washer that is the most important thing.i used a os91fx and still had to add 1 ounce to the tale,i have my balence set 1/2 inch father back than what is called for it hovers pretty good with the balance set like that.i even put my battery's on the firewall just past the servo tray aft,they are 5cell 6volt.
hope this helps some