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Derailleur moves the chain but won't stay in gear
Hi, I have an older road bike. It's a 12 speed with the gear shift on the lower bar. The rear derailleur seems to work, that is it shift from the small sprocket to the largest, but it will not stay in anything beyond the the small one. I have replaced the cable and adjusted the high/low as in the video (thanks) but nothing is working, it will not stay in any other gears.

Is the derailleur just popping back down to the smallest gear as soon as you let go of the shifter? If so, there's a little bolt on the outside of the shifter that screws it into the frame. Try tightening that up a little. It should be tight enough that the shifter won't move on its own, but loose enough that you can shift easily. Not a bad idea to open it up and grease the parts also.

I agree with Dave… Sounds like you may have friction-shifters? When you shift to a lower gear (larger rear sprocket) your tension on the gear cable allows this. However, the cable then slips back down (higher in gear terms) a bit. The reason it may stay in high gear (smallest rear sprocket) is because of the correct ‘h’ screw setting. You could see if it stays in the second-highest (second-smallest) rear sprocket if you tighten the ‘h’ screw…
Can you keep it in a gear with constant (hand) tension on the shifter? If you can shift I reckon the answer sounds like your friction shifter(s)(if you have them?) need a little work…

I have a similar problem, but it is with the front derailleur on a mountain bike. The cable will move it to the largest ring, but it won't hold it in place. I have not done a thorough inspection yet. Any ideas what would cause this?

You should take a look at the front derailleur tutorial. The derailleur settings might be off. Another possibility is, that the shifter is broken (or dirty). There is a ratchet mechanism that "locks" the gear in place. Cleaning and lubing the shifter might remove that problem.

Thanks Joe. I saw another post were someone cleaned theirs with WD-40, re oiled and it worked. The said the parts can gum up when idle, and my bike has sat idle for a while. I just started riding again because my son started.
I had some 3-1 I tried w/o cleaning. No success yet, so I will try the cleaning next. The cables feel fine. The components are 1990 Shimano Exage 400 LX. I know that sounds old, but everything still looks good on the bike - Trek 850.
I don't see a ratcheting mechanism that click, but I will keep playing with it before I give in to taking it to a bike shop! The mechanical engineer in me does not want to be defeated.

Progress. I believe it is my shifter that is the issue. I sprayed it full of a degreaser (liquid wrench) and then added a lubricant. If I hold the shifter in for a little while, it is taking hold on the 2nd and 3rd ring now. So maybe it will work itself back into good working order. I was unable to pop it open like they have shown in the tutorials or youtube videos. Anyway, so far, so good!

Things are working better this morning. I can shift up to the large gear with no problem. On downshifting, it jumps from the large ring to the smallest ring, so an adjustment will probably be needed.


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