Joined: Jan 2010
Posts: 9,835
Likes: 1,816
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
Can you first make any chain tension correction by moving the axle in the dropouts?
A picture or two might shed light on a possible derailer spring preload problem, as Simplex mech's often come up damaged in this way.
Some riders find the big-big sprocket combination extremely useful, but even if not there should be enough chain length to easily and quickly shift to and from the largest cog while in the big ring.