Y6B right yaw issue

Hi all,

First post here. I made a DIY Y6 copter. It was powered by an APM2.6 before but for some unknown reason it crashed from about 3m high 2 years ago. I have decided to repair it now and have installed a Pixhawk instead with AC 3.5 on it. I noticed and changed the configuration of my motors to a Y6b, frame type changed, motor rotation and propellers direction checked. I can take-off, loiter, more or less ok.

ISSUE: it has left yaw authority, but almost zero right yaw authority. Almost cannot stop my left spin. Something in the setup must be wrong.

Any help would be appreciated!