In the "slackest" gear (small chainring, smallest cog), check cable tension. If it is too slack, either adjust the cable tension ferrule if there is one (mine is at the downtube shifter mount), or pull the cable tighter at the derailleur.
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1989 Schwinn Paramount OS
1980 Mclean/Silk Hope Sport Touring
1983 Bianchi pista
1976 Fuji Feather track
1979 raleigh track
"I've consulted my sources and I'm pretty sure your derailleur does not exist"