That's strange. I basically ran the same setup in mine but with two ganged servos for the rudder and an on-board glo-driver (the battery for this was on the CG) and it balanced perfectly. Later I had to add a little tail weight to get the plane perfect. Are you using the stock fuel tank in its intended location? I do have my receiver behind the tank with the battery. 2400mah 6V I'm using Futaba S3305 servos in the tail so that's a total weight savings of .8 ounces. I suppose with that long of a moment to the CG it would make a difference but I can't imagine it would be that much.