Joined: Jan 2008
Posts: 23,639
Likes: 1,106
From: Mans of NC & SW UT Desert
Bikes: 86 Katakura Silk, 87 Prologue X2, 88 Cimarron LE, 1975 Sekai 4000 Professional, 73 Paramount, plus more
+1 Gunked up. Under that knob is a spring. If you take the derailleur off the bike, you can clean it and carefully adjust the spring. If the derailleur is still OK, it should return to the furthest back position, keeping the chain taught. Of course, this assumes chain is the right length.
But someone with more experience could probably correct my post.