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.

Doubtless it is the shifter as the derailleur simply does what it is ordered to by the shifter. When you shift and the shifter lever clicks, it should hold the derailleur in place. If not then either the shifter or your setup is the cause. Here is a section of the installation instructions; it shows the cable attachment. It also explains how to set that "stops" then move the shifter to the lowest chainring and attach with no cable slack.



Edit: Too bad you don't like the Sora shifters. Every bike I have ever worked on with the Sora shifters (old style with the thumb tab), have been very reliable and I have never had to replace a set. This compares to the Tiagra shifters which suffers from the Shimano slipping shifter syndrome (SSSS) after what seems a short time (I have never compared the mechanisms but I suspect they are just different enough that they are not as prone to SSSS). If the problem is the "new to you" Tiarga shifters, then just throw the Sora Shifters back on and see if they still work for you..I'm betting it will solve the problem with the front shifter.

Sounds like you need to fix the barrel adjuster issue.

Also be sure the shifter is in its lowest position with all of the cable out of the shifter when attaching the cable to the derailleur.

Does the shifter actually click into the new position? Is it possible the high-limit is preventing the click from occurring, in which case the chain would back down the moment you released pressure on the shifter?

If all else fails, release the cable and work the shifter. If you can't get it to click through the gears, spray some WD40 in its guts.

Thanks for the suggestions. I'm about to conclude that the shifter is the problem (I already terminated the new cable so until I find more, I can't easily remove the new shifter and cable and try the old one). I get 3 distinct clicks shifting both up and down, but the derailleur only moves when I shove hard on the upshift- and then moves right back. As soon as I shift down (the derailleur not actually moving), cable tension loosens noticeably (very slack). So I think the shifter is not indexing properly. So much for a cheap upgrade. . .

It would be weird, but I guess I could keep the Tiagra shifter on the rear and put the Sora back on the front (living in flat Indiana, I rarely shift the front anyhow).

This is significant. There should be no slack in the cable. (See the second part of post #3 above)
Disconnect the cable from the derailleur. Pull the loose cable out of the shifter while working the small shift lever several times. Connect the cable to the derailleur being careful to route it correctly at the fixing bolt and under the bottom bracket (through the cable guide). If the limit screws are set correctly you should be able to shift into the big ring.
I agree that hosing the inside of the shifter(s) with WD40 may help shifting performance.