So let's assume I'm on 1st gear.

Shift down to second works, third works, fourth, fifth, sixth all work.

Does not shift to seven...except on a rare occasion.

Now shifting up it gets weirder.

Let's assume I tried to shift to seventh, but it's still in sixth gear.

Shift up to sixth, gear stays in sixth.

Shift up to fifth gear, gear stays in sixth.

Shift up to fourth, gear moves up to fifth.

Shift up to third, gear moves to fourth.

Shift up to second, gear shifts to third.

Shift up to first, gear skips second and goes right into first.

Then I can shift down gear by gear except it never reaches seventh, and all the above can be repeated.

Also, when shifting up I sometimes have to coax it by slightly overshifting to the next but not enough for the shifter to engage - i.e. halfway to third from fourth and it'll shift to fourth.

Tried messing around with the limit and tension but I'm not certain I'm picking what's right. Headscratcher.

Suggestions?

M.

Sounds like classic cable issues. Friction somewhere between the lever and the der. Andy.

Try "resetting" all your adjustments. Screw barrel adjuster all the way back in on the shifter, along with the barrel adjuster on the derailleur, and unscrew the high and low adjuster screws.

Shift to first gear, but be careful not to let the chain shift past and into the spokes.

Set the high screw limit so the chain won't go past the first gear.

Shift down to 7th and set the low screw limit so the chain won't go past 7th gear.

Try shifting through all the gears. If there are still problems, shift back to 7th, unscrew the cable clamp and pull the cable with just enough force to get the cable moderately stiff.

Tighten the cable clamp and shift through all the gears again. If you still experience problems, use the barrel adjusters to get it just right.

Hope this helps...

Josh

The OP inferred that his cable tension/index coordination was good because in one direction all shifting was spot on (until the last shift where the der's return spring is at it's weakest). Yet when he shifted in the other direction the der didn't follow the lever's command. The most common way to reconcile these two behaviors is with cable issues.

Of course there could be other problems like a bent hanger or cage, mismatched cog count or c-c spacing, or a lever that has a gummy barrel movement (not a ratchet one). Andy.

I should add that in the corrective action (with the cable/casing) the OP should rebase line the system. Like Josh said. Andy.

I'd be curious what bike this "system" is on.
If it's a BSO, that shifting may not be that abnormal

Sounds very like a bent hanger to me. It should always be the first thing to check when there are shifting problems.

You can confirm it's cable friction by manually assisting the derailleur's return spring. If it shifts fine with you helping it, it's not likely to be the hanger.

The hanger will be bent, they pretty well always are. Whether it's bent enough to be a problem is hard to tell other than by straightening it. I know of no way of doing this other than by using a hanger alignment tool.

First thing to do, however, with mysterious shifting problems is to change the cable, inner and outer. Unfortunately OP will also have to reset limit screws etc because he's dicked about with them.

That's what I think too.

A bicycle is a simple machine but the devil is in the details. For mysterious shifting problems, check to be sure everything is in the default condition. That includes cable and housing condition and derailleur hanger alignment.

Possible causes of shifting problems :

1. Rusty shifter cables.
2. Kinks in shifter housing.
3. Correct housing length - too short or too long will affect the shifting.
4. Do not use brake housing for the shifters, they are too flexible.
5. Shifter housing that are frayed.
6. Bent derailleur hanger.
7. Rapid fire shifters that have old grease that sticks to the ratcheting mechanism.
8. Rear derailleur spring tension that are stuck.
9. Rusty chains with stiff links that skips.
10. Rear derailleur rollers that are covered with dirt and old grease that prevent it from spinning smoothly.

All these +1

With no pictures or bike data, sounds like it might be time to lube the cables, per Andrew's first post.

The symptoms really do sound like a classic cable "hysteresis loss" issue between shifter and derailer, but might also be due to too large of a chain gap between the sprocket and the top roller on the rear derailer which is adjustable on the better majority of derailers.

Try shifting the bike by pulling just on the cable...no shifters...if it shifts like it is supposed to..well check the other things mentioned...

You have two different things going on. Of course when the shift to seventh fails you will need two shifts to go back up, as the lever still starts out in 7th. After that it's operating pretty normally, with the exception of 1st gear, where it's common to be able to pull the derailleur a little more. The reason it won't get to 7th is (as mentioned) cable friction or a bent hanger - or both, and the already mentioned low spring tension by the time you get down to that end.

So you need to start by confirming hanger alignment and then isolating and correcting any friction problems, which could be lubrication, contamination, rust, kinking, etc. Then you need to do the previously mentioned "reset" of the tension adjustment.

I've been taught that bent hangers most often exibit a chain jump across cogs. Not what you've seen?

TIA

Not really. A bent hanger can cause inconsistent changes both up and down despite any amount of indexing adjustment. The problem is that it is sometimes very difficult to spot as it can be off in both the vertical and lateral directions. With any gear problems I've trained myself to check the hanger before doing anything else.

Great, thanks. We have the alignment tool at the coop where I volunteer, and I have used it several time to correct shifting issues not related to derailleur indexing. I have no shame in asking questions and not paying for any more lessons than I have to.

At least by starting with the tool you will know you have two set points, the alighment and index screws. I like to make sure the derailleur is clean and lubed too, espicially with index shifters. Extra friction is not my friend.

Thanks for sharing.

All true but hardly complete, and diagnosis is not a multiple choice procedure. One needs to look first at the items that are most likely the cause of the specific problem, eliminating them by means of isolation and substitution, rather than just going down a list.

You've been taught incorrectly. A hanger that is only bent in (the most common occurrence) will mostly just move all shift points inward and create somewhat more noise. If the hanger (or derailleur/cage) is twisted, however, the rotation of the pulley cage will cause misalignment in varying amounts due to it no longer rotating in the same plane as the cogs, and then may cause skipping gears, misalignment in some gears, etc.