Well, at least now I don't feel so bad about not being able to figure it out. Think it is a chain clearance thing, maybe. When I release pressure on the rear der. "arm", it stops, so I think the side plates of the chain may be slightly rubbing the next cog. Don't know why-not a bad angle, especially with largest cog front and smallest rear cog. Might try a small spacer between 8th and 7th. What have I got to lose?