Looking for some chain help. I seem to be in between link sizes—as it sits on the bike the chain is a little loose and a test ride confirmed that there is some occasional slippage. When I removed a link it was too tight and was not shifting properly. Is the solution as simple as removing a link in the chain and bringing the stoppers up a bit or is this a derailleur issue?

I'd suspect the derailleur isn't tensioning the chain as it should.

Was the RD recently 'serviced/rebuilt'? I can't 100% remember, but I seem to recall there are two hole 'positions' for the pivot spring, one providing more tension. I think there's a Simplex rebuild video on YouTube by a french bike mechanic that covers it. Also looks like you should shorten up the shift cable housing a bit- it's coming out of the stop at a pretty extreme angle....

that derailleur is 40+ years old and they were only marginal back in the day.

Looks to me like the tension spring is pooched out. Slack. Kaput.

I'd suggest a new RD. Get a mid range shimano off ebay for 20 bucks.

Caution - you may have a french threaded dropout. Solvable problem

/markp

Looks like a PX-10, so swapping out for a 'new' RD will be a little complicated- you'll have to tap the mounting hole and grind a stop into the hanger. Or (I think...) you could look for an old Shimano Crane RD that I believe doesn't require the stop (but I think would still require tapping).