I had to replace the barrel adjuster and shifter cable for my rear derailure. I screwed the bew adjuster all the way in, put my bike in "7" (all the way) with the chain on the smallest cog. I pulled the cable tight and secured it with the bolt. Now, it will shift up and down in the smallest gears fine but if I get her onto the big rings, the cable just stays slack and the bike doesn't downshift. WHat is causing the cable to go all slack like that?

Ken

Check to see that the inner cable isn't binding anywhere...

When you reset the cable turn the barrel adjuster out 3-4 turns before you snug up the cable to allow for adjustment up and down as screwing it all the way in does not allow you to slacken the cable during set up.

Without any cable tension check to see that the derailer isn't the problem and is working smoothly. Sometimes a little corrosion or even a little debris can get in their and keep things from working properly.

Since everything has to some apart check the shifter is working properly too.

Start from the shifter and work your way towards the derailleur

1) Cable head seated properly in shifter?
2) Cables properly routed? Through BB guide?
3) Cables dirty? Kinked? Housing dirty? Not seated in their stops?

Let's assume that the everything was working fine before you supposedly removed the inner cable from the shifter to fix the barrel adjuster, and nothing else changed.

Do you have a low normal (Shimano calls them "Rapid rise) derailleur by chance?
If it is low normal, you reverse the procedure.

When you attach the cable to the derailleur the shifter needs to be in its smallest cog position with all of the cable out of the shifter. Flip the smaller shift lever several times while pulling the slack out of the cable.
If this doesn't fix it you may have a bad shifter. You could flushing out the shifter with WD40.

Before you start hosing out the shifter think about what is happening. The cable is going slack ... that means the shifter is feeding it out (like it should) and the derailleur isn't taking up the slack. Since it shifts okay up to the larger cogs obviously it's not a low normal derailleur because given the installation procedure described it would only try to shift to higher gears and never move from the little cog. If the derailleur wasn't pulling down to the little cog you'd have noticed that when the cable was disconnected and since that wasn't a problem before you removed the cable it's unlikely to be the problem.

As operator said, it's most likely to be the cable snagging on something. Basically anywhere the cable is slack (or beyond all the way back to the shifter) isn't the problem. Look for where the cable is still taut and see what is binding it. If you have no tension at the clamp point of the derailleur then the derailleur might be the problem, but as pointed out before that's less likely. The quickest approach I can think of would be simply undo the cable clamp and (1) verify that the derailleur now pushes the chain back to the little cog and (2) feed the cable back and forth by hand and see if it appears to be sticking anywhere. Likely culprits will be either the inner cable has a kink or flat spot on it or the housing has a bur on the end where it was cut which is dragging on the cable.

OK, I'll get in there tomorrow and look for stuff gunking up the works. This bike has shifted fine for years. Last week I was riding it and it started shifting sloppy and then would not shift out of the big rings. I got it home and immediately saw that the metal of the barrel adjuster was ripped apart so I assumed that was the problem. I had to swap out cables too and I installed a new adjuster and cable. Same symptoms: the bike shifts around the small cogs OK but get her into the big ones and there she stays! The cable is slack all the way up the down-tube yet the derailure stays all the way left. When the cable is disconnected the derailure swings all the way back to the right. Humm.......

sounds like boken strand in shifter cable stopping it from sliding through housing from chainstay to rear der. Install new cable but crazy glue strands together at end before threading through housing.

Low normal derailers have a reversed set up so they default to the biggest cog and pulling the cable brings the chain down to the smallest cog.

Thinking that in this case that, if the cable is new and was not damaged the housing needs to be looked at... I really suspect that a worn rear housing is the most likely culprit.

You can check this by seeing of the cable pulls through this section of cable smoothly.

Woould also like to see a side shot of the rear to make sure the rear housing is not too short and causing issues.

I cut a little off the housing's end and lubed the inside of the sleeve. So, now it works, but not well. It does not shift cleanely or even one gear at a time but it DOES shift up AND back down again.