Upper chain roller:
I didn't much believe that the Suzuki engineers could make this mistake, so I had ignored mine. But I wanted a roller bearing one. While installing the new lower roller, I was surprised that, after cleaning, the original roller showed zero
wear after 15,000 miles, other than abrasion; but the upper roller had chain grooves worn in it. WTF, the lower roller does all the work, doesn't it?
So, I reinstalled the stock upper and new lower roller; but now I'm watching the upper closely. After a bit, I start noticing that, under G-load compressions (not the hard dirt hits, the highspeed highway dips), I thought that I was feeling a 'growl' under compression; only once or twice a ride, so easy to forget about. Hmmmmm, I wonder if that's the upper roller?? So, I took it off. That was several hundred miles ago, and I think that was the problem. Season is over, so I won't know for sure til next year. :(
Another potential upper roller issue: I originally planned on replacing it with a roller bearing, but they're a larger diameter; which would increase the interference.
Note: I was running a 15T front sprocket (stock 41 rear) for most of the mileage (when the groves were cut), but I noticed the 'growl' after installing a 16T, raising the chain toward that roller.
Edited to add MotoScooter's pic:
If you want to keep the upper roller: