Thank you for your replies fellars. My set up is the same as n9331v and I have come to the same conclusion.I found with full tanks the c of g moved back 5-6 mm so I have added 58 gm in the nose. The reason for my post was a problem on the test flight about 2 weeks ago. After 2 laps of the field at half throttle to trim, the aircraft was positioned for a 30 degree climb with full throttle after which it increased to about 60 degrees with no further control imput. It dropped a wing and entered a spin of 2 turns pulling out about 15 from the ground. Needless to say I landed immediatly thinking something had shifted back but nothing was evident. I have not attempted to fly it since. I just find it strange that the instruction manual tells you to balance with empty tanks ?