drmcgarry

Hi There, and thanks for your time in advance!

I recently had a chain/cassette/rear derailleur replaced and adjusted at a bike shop, the first few weeks appeared to work well.
Then when changing gears the chain started skipping or not engaging with the middle 3 or 4 rings.
The mechanic re-adjusted and said it was fine. Again on another tour, the chain is skipping,
I have checked every resource possible but cannot find a clue, other than a possible
mismatch in parts. Any ideas would be helpful!

Chain - Shimano Narrow SH JK [correct length, confirmed at bike shop]
Cassette - Shimano Hyperglide, 7 rings, 12T-32T
Rear Derailleur - Shimano Deore [adjusted]

Cheers!

Retro Grouch 

Here's a couple of semi-common possibilities:

1. Bent derailleur hanger. Shift into a gear combination that makes your derailleur arm point straight down. Stand your bike up straight against something and examine your derailleur from the back. If the derailleur arm seems to be pointing toward the rear wheel, that's it. With a 7-speed cassette I can usually "eyeball" it back into alignment. If you try to bend it back by yourself, leave the derailleur in place so you don't ovalize the mounting hole.

2. Bad cable housing end. Check all of your cable housing ends. If you see 1 or 2 stray wires sticking out, that's it. Your housing is gradually shortening itself and affecting the index shifting. It'll work fine for a week or two right after a tune up but gradually drifts off again. Cut the housing off square and install a new end cap and you'll be good-to-go.

cny-bikeman

No mystery - mainly a a lousy mechanic, who should have known to check for typical causes of inconsistant shifting, not merely adjust the derailleur. One time perhaps can be forgiven, but you should take it back and indicate the problem reoccured, exactly what gear combinations in which it occurs (both front and rear) and ask him to specifically check for hanger alignment, derailleur damage and cable/housing issues. Misalignment is not always easy to eyeball. The hanger just being bent inward does not often cause inconsistant shifting. It's usually side to side misalignment, twisting/damage to the derailleur itself, or hangups in the shifter/cable/housing.

Homebrew01

Tight link is another possibility. Pedal backwards slowly by hand and watch the chain as it threads through the pulleys & cogs.

jolly_ross

Mechanic may have replaced the derailleur cable (possibly just the inner) in the first instance. This would explain the derailleur working for a while then going out of whack.

Looking at your rear derailleur, from the back, where the cable goes into the derailleur, there should be a black plastic barrel adjuster - like a little dial. It turns with a clicky motion. Looking at it from from the back - turn the adjuster 2 clicks anticlockwise. Keep count! If nothing happens then return the adjuster to where it was before and take back to shop. If it gets better but still happens a bit try one more click anticlock. Whatever you do keep count so you can return to original position before getting the mechanic to sort it out for you. hth.

ultraman6970

To me sounds like a problem in the B-screw. That needs to be re-adjusted.