This may be a dumb question but I have never run into this before. I have an 81 Miyata 1000 with a suntour cyclone derailer, 5 speed freewheel and suntour barcons. The issue is that while riding it will not shift into the highest gear (smallest sprocket). When I put the bike on the repair stand it will shift into the highest gear. I made a few adjustments but get the same results. It's got me dumbfounded, but that's not too hard for me.

Mutt