Long story short, I have a new Shimano front derailleur that, when I shift to a larger chainring, is immediately pulled right back to the small ring by spring tension. The derailleur is a FD2203 (2300 series, braze on but with a clamp to convert to clamp on)- which was the only available replacement for the 4403 Sora triple that originally came on the bike (spring stop broke on it, so I had no choice but to replace). I noticed that the new derailleur has an odd cable attachment point, and the cable has to bend about 60 degrees which can cause the cable to rub on the derailleur body. Most importantly (I think), it is hard to keep decent tension on the cable. Complicating this is the fact that the barrel adjuster boss on the down tube has corroded, and I have maybe a half turn at most from full stop before it just comes out.

At the same time as replacing the derailleur, I "upgraded" to Tiagra shifters (hated the Sora shifters)- but purchased them used off eBay. Could a bad shifter be the cause, or is a lack of cable tension the probable culprit? I've also installed all new cables and housings.

I could take it to an LBS but am trying hard to do this myself for a change. . .

Thanks.