Had that same problem just last week. The right motor was dirty from the dust of the brushes. Older motors don't have any openings for that dust to escape and builds up. Blowing out the brushes and checking that they are still good usually does the trick. That is, open every brush cap and remove them all, then use your air nozzle from your compressor and blow it out good. If it's too bad you may need to open the motor itself to get the dust out.

Those motors can cause all kinds of weird situations with the WAV. I once had a WAV where the throttle turned the unit and the steering made it go forward and backwards! I took out the motors, opened them, blew them out real good and put them back in and it worked fine!