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I have recently been given a bike, all works fine accept the gears keep jumping down to 3. It'll say it's on gear one on the handle bars, but the chain is sitting on the third set of teeth. So I'm looking for a replacement derailleur but I cannot seem to find the right one. It is a Shimano SIS; I have attached a photo of it. If anyone could help would be much appreciated (:

enter image description here

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    What does "it'll say it's on bear one" mean? "Gear 1?" It sounds like a simple cable adjustment to me. – Transistor Sep 9 '17 at 9:51
  • When you say 'gears keep jumping down to 3' does that mean you can initially get the chain into first gear on the largest sprocket, but then the chain moves back to a smaller sprocket; or that you can't get the chain into first gear at all? – Argenti Apparatus Sep 9 '17 at 10:28
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    Have you (or anyone else) tried to fiddle the two limit screws? I can only see one in your photo, the silver one poking out below the I of shImano. There should be a second one, perhaps it has dropped out ? Between a missing limit screw and possible cable adjustment, this could be a cheap fix. – Criggie Sep 9 '17 at 11:03
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    It probably just needs proper cleaning, lubrication, and adjustment. And the missing limit screw. – Daniel R Hicks Sep 9 '17 at 12:03
  • Read this article on rear derailleur adjustment. If you're not willing to do it yourself, it should be a quick and inexpensive adjustment at any bike shop. – Batman Sep 9 '17 at 13:58
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You need to tighten the cable. Also, check that you're not missing a limiter screw, because that looks plain dodgy.

Once the limiters are set up, follow these steps:

1)Set the gear shifter in the highest number (6/7/8 whatever your maximum is).enter image description here 2)Undo the bolt pinching the cable, the bolt with the spanner on it. This will allow the cable to move freely (if it's not seized up and if you've loosened it enough. But, don't let it go all the way through! enter image description here 3)Then push the derailleur manually to the biggest cog whilst turning the pedals and wheel. Push on the 'O' of 'SHIMANO' on your particular derailleur, and it will move the derailleur, and in turn, chain onto the biggest cog. enter image description here 4)Allow the slack of the cable through and make sure it's sitting in the inset. Then set the clamp bolt down tight. About as tight as you can with a small 1/4" ratchet from the head of the ratchet. Note, it'll work the same with your nut-style pinch clamp like this pinch-bolt style. enter image description here 5)Then, let go of the derailleur and hopefully it'll drop down near the smallest cog. In this image, this is what the bike mechanics where I work call 'Non-Limited'. enter image description here
(source: mountainbiketoursthailand.com)

If it DOESN'T sit on the smallest or second-smallest cog, keep repeating steps 2,4 and 5, pushing the derailleur up to the second largest cog, and then the one next to it each time until it sits on the second cog when you let go. If when you let go, it goes to the second smallest cog, screw in the barrel adjuster until it aligns with the very smallest cog. Then proceed to tighten and loosen it as you need until the gears shift into each cog accurately and easily. enter image description here

Very good luck, and if you cannot do this yourself, give it over to a mechanic. I'd personally give it a go if I were you. If you manage it, great! You've saved yourself some money. And if you mess it up, there's not much that can go wrong, nothing is going to really break. You'll just be stuck at square one and need to take it to the mechanic, which is the position you were already in anyway. :)

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    Nice answer! That first picture of the shifter in 8th is misleading because no screw needs to be removed. I'd suggest something like d3d71ba2asa5oz.cloudfront.net/12012911/images/ld8772.jpg as a replacement. – Criggie Sep 9 '17 at 20:35
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    Oh god, I hadn't even noticed that! :O I only picked it because it showed the gear selected to it's maximum, hehehhe! I'll try and find another now, hehehehe. – yollooool Sep 10 '17 at 11:23

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