fadetoblack6902

I have an old mtn bike which has 8 speed xt rapid fire shifters, and a 7 speed freewheel. It had an stx long cage derailleur, and it shifted fine with the limit screws set correctly. I just switched in a short cage XTR 8 speed derailleur. It shifts fine from the smallest cog up until you get to the 3 to 4 shift. during that shift it requires two clicks of the shifter to shift up to 4, and the same going back down to 3 from 4. but other than that it's fine.

The derailleur says it handles up to 32 teeth, my freewheel's largest is 28. Could the short cage be causing this? any other ideas?

Tapeworm21

Shift into your highest gear, (smallest cog) and pull out the slack in the cable. Either that or loosen the low adjust. Hard to say without seeing it.

Retro Grouch 

7-speed and 8-speed spacing are close, but not exactly the same. I've tried mixing and matching various 7&8 speed components but have never been able to get them to shift to my satisfaction for a very long term. I've gotten the best results by adjusting the cable tension with the chain is in the middle cog position. That way the cable pull difference is split in the two different directions.

miamijim

In addition to Retro's comments...are you running a freewheel or a freehub? If your runnig a freehub you can throw 8-speed spcers in between the 7 -speed cogs.

tellyho

How about up front? I'm assuming you have three chainrings? Sometimes a short cage RD cannot take up all the excess chain in the small / small combination.