Originally Posted by Ken59
weak rear motor
Low battery voltage under load could cause the tail motor to not reach maximum speed and the main motor is by far the heaviest load. But then the main motor is the first to slow when battery voltage drops.
My DH9104 rotates quite well in either direction at full throttle. Will have to take note if there is a change when the battery runs low. I don't remember any tail problems with a low battery.
If the tail gears don't have a lot of slop (watch the internal coils from the rear while turning the blades), then it likely is the tail motor, specifically the brushes. Take the tail motor apart after you replace it and see what is in there. A good cleaning around the commutator may fix it. I'd like to know how dirty the motors get.
Stormforce took apart a main motor and found carbon brushes. He was expecting simple metal spring contacts.
Try a drop of oil on the bushings when you can get at them.