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I replaced the rear cog as the chain I had snapped and I thought it had taken some teeth off. Now when I pull away in 1-3 gears it's ok, but if I am in gears 4-7 and I try to pull away ( say I have stopped at lights and not had chance to change back up to 1st) then the chain slips and bounces off the cog its on. If I change up through the gears when I am already riding it isn't a problem, just seems to happen if I put any tension on the chain in the higher gears when I have stopped and start pulling away again. Could this be a cog problem? set up or chain problem? Its done it for a while now and I have put up with it, but I am overhauling the bike and would like to have a heads up as to what I should be looking at to fix/ replace.

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    Did you put on a new chain and are you sure it is a correct chain? – paparazzo Dec 3 '15 at 9:43
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    Check the chain for a stiff link. – Carel Dec 3 '15 at 11:23
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    You may try to find out whether the problem you are describing is actually the chain "jumping" or the freewheel/freehub slipping. If you are only experiencing the event as "the pedals suddenly jump forward" and aren't actually sure which item is moving free, you may want to narrow that down first. – Deleted User Dec 3 '15 at 17:23
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    Voting to close. New user that has not checked back. Need more information to properly answer this question. – paparazzo Dec 4 '15 at 9:31
  • You said that you replaced the cog, did you replace the entire cassette? What kind of chain and cassette did you use to replace the original? – Benzo Dec 6 '15 at 23:28
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When you replace the cluster (rear cogs) you almost always need to replace the chain too because they wear together. If you put an old chain on a new cluster, you often experience that kind of because the length* of the links in the chain is slightly longer than the spaces between the teeth on a new cluster.

*Note the links don't actually get longer, it's just that the rollers wear out and allow a little play and the links sit a little further apart. This is known as "stretch" though it's different to the way something like elastic stretches.

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    He broke the chain - its a fair assumption the chain was replaced too, but not guaranteed. Good point! – Criggie Dec 4 '15 at 4:07
  • A new chain on a worn cassette will definitely cause this issue due to the wide pitch of the old cassette and the narrow pitch of the new chain. A new cassette and worn chain will likely have issues like this as well, and it will quickly wear down the new cassette due to the wider pitch of the worn chain versus the tight pitch of the new cassette. – Benzo Dec 6 '15 at 23:31
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Look at the chain closely as it passes over the cassette. It should be even, not with gaps under the chain before/after the contact point.

I'm now wondering if you've got a mismatch between the chain and cassette. Is the chain a 6-8 speed one of the same width as the old chain?

edited because I didn't read the initial question properly

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