Thanks in advance for the help. I thought that I had this one solved, but it is a recurring problem on my wife's touring bike, and I need a lifeline here... Long description, but I want you experts to know what we're working with...
The bike is a Trek 620, which I have set up with a new SRAM 9-speed 11-32 wide range cassette, a Shimano Deore XT reverse action 9-speed rear derailleur, the original Sugino AT triple crankset with new rings (48/38/26), Shimano 9-speed bar end shifters, and new Shimano cables and housings (the housings are silver, and very firmly seated). The only barrel adjuster is on the derailleur, as I had to add clamp-on cable stops (no downtube braze ons) so they're just plain stops. The chain is a Shimano 9-speed. All parts are new, except the shifters, but they're in excellent shape and shift perfectly when the bike's on the stand.
I set the B-screw according to the derailleur instructions and the gap between the top pulley and the largest cog appears to be correct.
There is little slack in the chain. So with the largest chainring and the largest cog, the derailleur cage is on a nearly straight line toward the front bottom of the chain ring. Taking out one more link pair doesn't appear to be an option, unless we want to risk an accidental big-big shift that would torque the derailleur...
Here is the symptoms, and I've had hours of riding behind my wife to observe this and can't solve it.
When she's climbing in the smallest chainring, and the climb is leveling out, she shifts from say the third largest cog (third position) to the fourth position. Or from the fourth position to the fifth position. In either case, the derailleur cage is nearly horizontal backwards taking up the slack that this creates. But the problem is that the chain wants to jump up to the larger neighbor, and does so periodically. Remember that this is a reverse action derailleur, so it indicates a cable that's too loose, right. So, I back out the barrel adjuster a quarter turn (and the derailleur cage moves imperceptably outward toward the smaller cog).
The jumping appears to be solved briefly, then later in a subsequent steep climb, she'll go to the first cog with the inner chain ring. As she shifts to the second position, it jumps to the third (cable too tight?).
The jumping between the fifth and fourth cog also happens from the middle chain ring... When I adjust the barrel at the rear derailleur to prevent one symptom, it causes another. The main problem is around the fourth and fifth cogs, which are from a wide range setup (11-12-14-16-18-21-24-28-32). Maybe the jump from 26(front)-24(rear), which is okay, to the 26(front)-21 or 18 (rear) isn't okay from a slack standpoint? Or is my problem really cable tension adjustment?
I'm about to pull my hair out. Maybe the cable housings are changing length somehow, by not being seated correctly in their stops? H E L P ! ?
I know you guys are the best and someone will have come across this before.