To adjust the timing you need to be able to rotate the end with the brushes. I've done timing changes to Speed 400 motors, but not anything larger. For our purposes, neutral timing is best - at least in my opinion. If you advance the timing for more power while going forward, then you lose power in reverse. The simplest way to get neutral timing to run the motor forward and then in reverse. When the speeds are the same in both directions then you are approximately neutral. Beyond timing the bearings and the seating of the brushes can also result in speed differences between motors. I would expect these variations to fairly low, maybe up to 5%?
The biggest problem I am having is with my starboard motor running faster then the port in forward rotation and the opposite in reverse rotation.
It sounds like your starboard motor has slightly advanced timing. Below is the tool I used for timing the Speed 400 motors. The steel pins in the tool fit into the holes on the end of the motor and give you the leverage to rotate the end without disassembling the motor.
