robkyle

Just replaced my mountain bike shifter cable & housing. But I just can’t get the shifting dialed in. I looked at a couple of videos on YouTube and didn’t really learn much. When I get my low gears shifting nicely, the high gears are out of wack. My limit screw settings were ideal before the cable change, so I’m reluctant to make a change there. Any suggestions? Thanks!

FBinNY 

I call what you're seeing "spanning" error. Meaning the RD is moving slightly more or less than the cassette spacing with each click. Over a few steps the error adds up and indexing fails.

Since you replaced the cable and housing the likelihood is that the issue is directly related to that. I'll assume that the RD isn't moving far enough for the moment,

First of all, did you use compressionless housing?
Properly pocket it in any and all ferrules, frame stops, and shifter body? Is the inner wire the same gauge as the original?
Are there any sharp bends which might add friction?
As you work the lever, do you see any movement of the cable (housing) as it taken on added tension.
And lastly, did you attach the wire to the correct side of the pinch bolt of the RD (most important)?

If you double check the above and all is good, please take a moment to assertain whether the RD is traveling too far with each click or not enough. Also, is the problem the same going inward from small to large sprockets as the reverse? Lastly, shifting big to small (return spring direction) does the problem seem to get progressively worse as you go farther, or is it more random, or only in certain gears.

These are things I quickly check when solving shift issues, but I can't see or touch the RD so I'm depending on you to observe for me.

Bill Kapaun

That's pretty classic for high cable friction.
Since going to a lower gear (bigger cog) PULLS cable, things are fine.
The delayed shift to a higher gear indicates the RDER return spring isn't strong enough to over ride cable friction. Bad enough and you might actually have to ride over some "bumps" to jar it the rest of the way.

Are there any details you omitted like I also installed a 12 speed cassette on my 10 speed bike or??

Iride01 

High gears being the smaller cogs on the rear with less teeth?

What kind of shifter? A mountain bike shift lever or STI? Regardless, when you attached the cable to the pinch bolt of the DR are you certain the shifter was cycled to release all the cable it could. And also check that all your housing ends are properly in all the ferrules, stops and any in-line adjuster or barrel adjuster. Might just need to adjust those barrel adjusters or in-line adjusters. If they are at their limits, then them put in neutral or middle position and then re-attach the cable to the DR.

As always, I recommend you go by the manufacturer's instructions. Shimano can be found here. https://si.shimano.com/en/#/

Since you have a 12 speed rear, there should be a DM for it if it is Shimano. Look for the DM and follow everything about it from beginning installation to adjustment. If you don't find the general DM, then look at both the DM's for your particular DR model as well as the DM for the STI or shift lever.

Moe Zhoost

This is so often the culprit.

cxwrench

It's new cable/housing so this is most likely not the problem. The OP didn't say it was 'bad going one way and good the other' he said it not indexing correctly at one end and working well at the other end. This is a classic symptom of not getting the cable anchored in the right fashion.

Yep, most likely this.