I've been having a lot of problems flying my Y6. Yaw would change while flying forward or backwards. The motors on the bottom would getting really hot. Someone at the airfield that I fly at notice that I had my props on upside down. He mention that even though the motors were upside down, the props still needed to have the raised letters facing up. I'm going to flip them tonight and test fly tomorrow.
For sure the bottom props have to be installed that the letters are on the upper side. You want thrust into the air and not into the ground. When you mount the props upside down, you get a lot less thrust and so the moros have to spin much faster.