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On a 3x7 gear 1990s Trek 800, the rear derailleur skips the second-smallest and second-largest cog, but goes into the smallest and largest ones. How to fix this? Shifting towards larger cogs, the derailleur puts the chain onto all cogs, but shifting away from the largest, both 1 and 2 on the indexed shifter keep the chain on the largest cog. Shifter 3 puts the chain on the third largest cog. Sometimes for smaller cogs the shifter number differs by 1 from the cog number (shifter 6 keeps it on 3rd smallest cog) but shifter 7 puts it on the smallest cog.

It may have been caused by my incompetent adjustment of cable tension after the derailleur started having difficulty going onto the largest cog.

Bike bought new, all original Shimano components, little used, kept in a garage. No rust. Haven't changed chain in decades but not much stretch (~1/16 inch per 12 inches).

Derailleur cable looks clean, no rust or dirt visible. No kinks or other visible problems. Rear derailleur pivots and hinges are moving freely. Pushing on each jockey wheel's edge moves the edge about 1mm. Not sure if that is too loose. No visible wear, unsurprisingly given the rest of the drivetrain is original and not that worn. Moving derailleur by hand or by cable pulling is easy - it goes into all the gears. As far as I can tell by eye, the derailleur is straight (both viewed from above and from behind). The 2-3 sections of the chain line up behind each other.

Looking for cheap or simple solutions first.

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    Put on new shifter cables and housings since they probably also 30y old.
    – Carel
    Nov 13 '20 at 8:31
  • Looking for cheaper/simpler solutions first, but thank you, @Carel. Cleaning and oiling the cables seems similar. Nov 13 '20 at 8:36
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    If you hang the bike off the ground, and slow-pedal with your right hand, can you use your left hand to move the derailleur across all the gears? (mind your fingers!) If that works, try the same but by pulling on an exposed section of inner cable. The first tests if the isolated derailleur can move as expected, and the second tests some of the inner cable but leaves the shifter out of it.
    – Criggie
    Nov 13 '20 at 21:51
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    Can you also inspect the jockey wheels for play and wear, and check if the derailleur hanger and cage hang vertically? A subtly-bent hanger can do weird things to shifting.
    – Criggie
    Nov 13 '20 at 21:52
  • Pushing on each jockey wheel's edge moves the edge about 1mm. Not sure if that is too loose. No visible wear, unsurprisingly given the rest of the drivetrain is original and not that worn. Moving derailleur by hand or by cable pulling is easy - it goes into all the gears. As far as I can tell by eye, the derailleur is straight (both viewed from above and from behind). The 2-3 sections of the chain line up behind each other. Nov 14 '20 at 6:58
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This sounds like a classic case of the shift cable hanging up in the housing. As the shifter releases cable increments to shift to smaller sprockets, the derailleur spring cannot pull the cable though the housing properly.

You can try removing, cleaning and lubricating the cable. Replacing the cable and housing is the proper solution though, and is relatively inexpensive.

While you have the cable detached, check the derailleur itself is moving freely.

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  • Derailleur cable looks clean, no rust or dirt visible. No kinks or other visible problems. I oiled it anyway. Rear derailleur pivots and hinges are moving freely. Thanks for the idea! Nov 13 '20 at 19:38

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