mattface 

I just switched the cassette, and shifters from Shimano 8 speed/Gripshift to Ultegra 9 speed/STI.

I got the derailure shifting and indexing pretty well down, that is from the small cogs to the large ones, but when I go to shift up from the largest 25 to the next smallest 23, it takes 2 clicks to go down 1 cog, then it shifts normally until it gets to the 3rd smallest (14T?) skips 13T all together, and goes straight to 12T. If I yank the derailure towards me (away from the wheel) after shifting, and before pedaling, then it shifts where it's supposed to, so I'm thinking the spring tension on the parallelogram is to blame, but I don't know how or if the tension can be increased.

It's an STX/RC long-cage derailure, and although it had been being used with a mountain triple, and an 11-30 cassette it's now used with a 105 double (53-39) and a 12-25 cassette. Anyone have any tricks I can use to adjust to shift up properly?

punchthemonkey9

I have a similar problem, on my 8-Speed when I'm in 6th, it takes two clicks to get to 7, and then I can't get into 8th. I also think it has something to do with spring tension.

Mothra

The two symptoms you've listed is a sure sign of cable-friction. Going up the cluster into lower-gears is easy because your hand easily exerts enough force to overcome the friction. However, in going down the cluster into higher gears, only the derailleur spring moves the derailleur. And if there's extra friction in the cable-housing, it won't allow the derailleur to move. So take your cables apart and clean or replace them. Be especially careful about the joint at the ends of the cable-housing. The ferrules should fit snugly and in a straight path for minimal bending of the cable.

Bobby Lex

My philosophy is to always try the simple fix, first. If no luck, then work toward harder fixes. The simple fix for shifting problems is this:

If you are having problems shifting from left to right (bigger cassette-ring-to-smaller-cassette-ring) turn the barrel adjuster on your derailleur a quarter turn in the same direction (right). Check your shifting. Repeat once or twice if necessary.

If you are having problems shifting from right to left turn the barrel adjuster in the same direction (left).

Sometimes that 2-second adjustment is all that is needed.

Bob

Flanderflop

If you just replaced the shifters, I would assume you just replaced the cable, which rules out my first instinct which would be to say cable friction, that is unless the housing was cut badly and is rubbing. A minor tension tweak may solve this for you as suggested above. If both of these options fail, you may be right that the derraileur is sprung, as happens with some old ones. There are some where you can adjust this spring tension, but I don't think that this is the case with the one you mentioned. If this is the case you will probably want to either get a new one or just deal with the issue.

2372ighost

If you changed to a 9 speed cassette, did you buy a new 9 speed chain or are you still using your wider 8 speed chain?

Retro Grouch 

That little short section of cable housing adjacent to the rear derailleur is absolutely evil. If you didn't replace it before, do it now and don't skimp on the length. I think that tight curve is where most of the cable friction comes from.