Why my Proto 2000 F7 has different speed in Forward and Reverse?

[trainsBuddy]

Hi, on one of my F7 units I noticed that it runs about 1/3 slower in one direction than the other. I'm running DCC. Ii is running in a consist with other 3 F7 units (same Proto 2000), so it ends up being pulled in one direction by the other engines, but is fine when going in the other.

What could be the problem? I figured  if anything mechanical, it would've run badly on both directions. Any suggestions? Other 3 units that I have don't appear to have same problem.

[Lee 1234]

Check for end play or binding in the motor, drive shaft and worm gear area. They move forward or backward ( as the locomotive moves) depending on the direction the locomotive is moving.

[trainsBuddy]

How exactly do I do it? Do I need to take the motor out, or just move wheels and look at the worm gear? Also why would it matter what direction the motor is spinning?

[Lee 1234]

Take the shell off and run the locomotive. Watch the worm gears and the motor shaft go back and forth when you change direction. Did someone leave out a shaft spacer?