I have one too and it tries to go to the left. Make sure the swash plate is level with the trims set in the center. I found that out of the three servos that control the swash plate one of them was pushing up too much because it wasn't centered. I felt that this was the reason that it tried to move the left when I add throttle. Being unable to ceter the servo through the Tx I turned the servo upside down and was able to center the control horn. This caused the link to have to be longer but it worked for me and now the thing hovers up strait.
If all the servos don't move the same as throttle is added the swash plate will tilt as it moves causing unwanted cyclic. Something to look at.