sirious94

Hello

So I don't know what is going on.

Front shifting works fine.

The build:
Shimano RSX 8s STI shifters
Shimano 600 RD
Shimano (something) Cassette
SRAM PG850 chain (the ones for 8 speed)

The problem:
When I shift up, I need to push up to the next cog (not all the way, just half the way) to shift. When i shift down, I also need a little extra bit. This is only a problem in some areas of the cassette. Main problems are the second to smallest cog is skipped, and everything else is one off the shift number except the largest takes two. Then shifting down it takes a bit before the chain goes down each cog.

How do i fix this? Is it the STI shifter? Could it be the chain? Is there a RD adjustment that I could make?

Thanks

EcoRacer

Things to check:
Cable friction can cause shifting to be really slow.
Is the derailleur hanger aligned properly?
Are the low and high positions on the derailleur set properly?
Is the rear derailleur straight and free of excess play?
Can the rear derailleur move properly, no stuck pivot points or weak spring?
Is the shifter compatible with the derailleur?

Retro Grouch 

I agree with Eco. It could be any of that stuff - BUT.

Whenever I encounter a problem like yours the first thing that I check is the derailleur hanger alignment and most of the time that fixes it. That's a fairly old bike so I'd be surprised if the hanger has never taken a hit that might have bent it a smige.

For a shade tree repair, shift into a rear cog that makes the derailleur arm point straight down. Then stand your bike up straight against something and look closely at the rear derailleur from the back. If the arm seems to be angled toward the wheel even a tiny little bit - that's it. On 7-speeds I can just bend it back with my hand accurately enough to make it index properly. 9-speeds, for me, require using a gauge. 8-speed is in the middle so YMMV.

operator

+1

1) Check hanger
2) Check that the cable is anchored at the proper position on the derailleur
3) Check cable + housing, disconnect cable from rear derailleur and isolate sections or just replace it all and remove any doubt
4) Do https://www.parktool.com/repair/readhowto.asp?id=64

Assume the derailleur is not bent and that the STI's aren't giving out.

Yan 

You might try backing off the b-limit screw as much as possible.

operator

Yeah blindly adjusting things always tends to work.

LarDasse74

+1.

I vote for adjustments wthout a plan. Post pics after riding.

Kimmo

If you rule out a bent hanger or sloppy derailleur, adjust the indexing so it shifts to larger cogs properly, and shift to smaller ones while pulling the derailleur to the right (assisting the return spring). If it shifts properly while you're helping it, then it's a simple case of too much cable friction for the derailleur return spring.

I had a similar issue with my shifting; no amount of adjustment would get it to shift properly both ways, despite cable friction seeming pretty acceptable. The return spring on my old 600 derailleur was pretty shagged out... the preload screw was already set to maximum too. So I just prised up the end of the spring and slipped a little nut over it, increasing the tension quite a bit.

Bingo, perfect shifting again.

mechBgon

Also confirm the cable doesn't have broken strands, particularly at the shifter end.

sirious94

So i tried swapping out the 600 shifter for an Exage one with a stiffer spring, and it works (on the stand at least) i am going to take it out for a ride today and see if it works.

Kimmo

You mean the derailleur, yeah?

If it's in good nick, I'd suggest finding some way to increase the preload on the return spring and re-employ it rather than the crappy Exage one.

...Although, I remember seeing a fairly unique feature on Exage derailleurs - a spring-loaded arm for the pinch bolt, allowing shifts to a larger cog before pedalling.

Thought it was a nifty enough thing to deserve not to be so rare...