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As of late I'm having some trouble with my rear derailleur.

When I shift specific gears (most likely to happen to the middle gears), the rear derailleur feels like it doesn't have enough power and needs another shift to change gears (when it does it changes two gears at once).

And when it does change 2 gears I have to shift one gear down again for my desired gear.

I have tried cleaning the whole thing as best as I could. There was a bit of gunk in the derailleur, so I carefully removed it and also cleaned the cassette extensively.

As always I'm going to consult with LBS anyway but maybe this is an easy fix I'm not aware of, because I'm too scared to touch any screws/parts myself.

Parts in question:

REAR DERAILLEUR SHIMANO RD-M390, SGS

I can't figure out what cassette it is though, but it mentions 11-32T on the parts list so I think that's the cassette "size" and it's 8-speed.

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    Bad adjustment, worn out cables, worn cassette and chain and worn pulleys can all cause this. – ojs Mar 26 '16 at 18:38
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    How many miles on the bike? Likely it needs a new chain, and a good chance a new rear cassette as well. – Daniel R Hicks Mar 26 '16 at 19:28
  • I personally suspect its a little bit of everything, also around 500 km on the bike since I bought it, cleaned after every few rides – zython Mar 26 '16 at 19:39
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    That's practically new. If there is warranty left, take it back to the store. – ojs Mar 26 '16 at 20:38
  • I'f you're too scared to touch the actual parts, then take it to your local bike shop. – Nic Mar 26 '16 at 23:13
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Only 500 km from new? Sounds like cable stretch where the inner cable increases in length from new as it beds in.

Did you use up your free service? Most new bikes from shops come with a 2-8 week "tweakup" to fix minor things like this. If you bought the bike off the web, then its up you to fix it.

Firstly check out your brake line - there should be some form of barrel adjuster, often at the deraileur end. Try undoing it a turn to tighten the cable (ie lengthen the outer), and see if it makes a difference to shifting.

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Sometimes they're on the downtube (this is more of a road bike thing)

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You shouldn't need to adjust anything else on your deraileur, so no screwdrivers or hex drivers needed.

This is easiest if your bike is hanging up off the ground so you can rotate the transmission. Don't put your bike upside down, things don't work exactly the same the other way up.

Also its probably time to lube your chain.

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    Cable stretch was my thinking too, other worn parts could do this but being as low mileage as it is i would put my money on cable stretch. It would make sense for skipping gears as well. – Nate W Mar 28 '16 at 18:31
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The general concept of adjusting rear index shifting:

If the derailleur is slow shifting to a larger cog then the cable is too loose (not enough tension). To increase cable tension unscrew (turn counterclockwise) the barrel adjuster.

If the derailleur is slow shifting to a smaller cog, then the cable is too tight and tension needs to be reduced a bit.

Generally it is easiest to adjust shifting by first adjusting the accuracy only with the three smallest cogs:

  • With the chain in the smallest cog, one click should move it up.
  • One click will move it back down.
  • Move to next to smallest cog and now repeat shifting to one larger and one smaller
  • If the chain is a bit slow moving up to the larger cog, then increase cable tension and if it is slow moving down to the smaller cog then reduce tension.

Once the shifting is working well on these three cogs then it should work well across all of the gears be it 6 speed or 12 speed. Campagnolo recommends using the middle cogs for tuning the shifting but it works fine using the small cogs on my bikes.

If the shifting gets inaccurate when shifting to the large cogs then two common issues are:

  1. The so called "B-Screw" is not set correctly. This screw adjusts the gap between the jockey pulley and the cassette cogs. If there is no clearance then the chain is literally being pushed into the larger cog. Too much of a gap and the shifting gets a bit sloppy.

  2. The derailleur hanger on the frame is not aligned (i.e. is bent).

Dirty cables (i.e. cable friction) tends to slow shifting from large-to-small much more than small-to-large. This is because the large-to-small shifts are accomplished by the derailleur's internal spring while the small-to-large shifts are powered by the rider's hand.

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