I seem to get bit by bad brushes a lot, and I'm not loyal to any brand. Maybe 5% or so? I leave them at the track for "bait".
It's more unusual for me I guess Dave, but it did cross my mind when the motor was giving me trouble, and I'm glad I checked! After the motor ran fine (well, actually finer than fine!). I started swapping the first pair of brushes in one at a time with the new ones, and found it was just one of the brushes causing the trouble. I looked at it as closely and carefully as I could and there's no way to see any difference between it and a good one I could make out. All I can guess is that, the single brush just wouldn't pass current slightly as well as it's mate. It didn't seem any harder or softer, and the color was the same as is the surface "texture".
***Anyway, copying Dave's "recipe" (50/29 on a .560" diameter arm with stupid-high timing jammed into a C can with the magnets honed way the heck out to .580") makes for a shockingly strong sounding/feeling/revving motor. I'm not going to suddenly be timing a bunch of armatures at 42 degrees, but it's cool to know the option is there. 
