First, put the bike in a bike stand and spin the cranks and simultaneously pull on the bare derailer wire on the downtube and see if it shifts. If it does, then it's most likely a cable tension problem. If not, then adjust the derailer stop.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1