any dirt in the commutator? Brush dust?

Have you done any parameter (CV) tuning on that decoder yet?

The ESU Lokpilot V 4.0 is quite advanced and can be tailored to properly control almost any locomotive. ESU provides suggested CV values for all of the most popular motors. It even has the ability to make a high speed run and determine the best settings on its own, assuming that you have a long enough straight track section for it to complete the run. It is all detailed in the great online documentation available to download from ESU's site.

now lets start from the beginning, have you bought the loco second hand ?, is it your own loco ? have you changed both brushes, have you cleaned the motor before replacing the brushes, have you looked at the brush plate and see if the brush holder shafts are 90° to the armature shaft. look at the armature and make sure the 5 poles are clean between the gaps.

another problem with older type motors the motor bogy wasn't always 100 % accurate and by changing the motor to a 5 pole highefficiency motor it may not always fit properly, by testing if the motor and armature are in line you should be able to turn the wheels freely with or without brushes (easier without brushes) and before this you could move the wheels without the the motor as well, to see if the cogwheels are ok in both directions.

regards.,

John