What could cause this...


Good: When I fly the Hexa XL without any payload installed except the camera mount, it flies just fine and hovers steadily.
Bad: But - as soon as I install the 7D to the mount, it starts moving to its left. At first you might think that the camera messes up the balance of the Hexa, but it is not the case. No matter where I set the CoG by moving the mount and the camera, it still acts the same way by rolling to the left.
I have also disconnected the mount servos from the FC board, but it didn't help.
Any advice?
Good: When I fly the Hexa XL without any payload installed except the camera mount, it flies just fine and hovers steadily.
Bad: But - as soon as I install the 7D to the mount, it starts moving to its left. At first you might think that the camera messes up the balance of the Hexa, but it is not the case. No matter where I set the CoG by moving the mount and the camera, it still acts the same way by rolling to the left.
I have also disconnected the mount servos from the FC board, but it didn't help.
Any advice?