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BACKGROUND NOTE: The bike is a 1992 Univega MTB that originally came with 6-speed SIS with a triple chainring. I removed the SIS shifter when it broke, and went with a friction shifter. The bike is used as an all-purpose commuter/errands bike; I was having no problems with the "old-tech" setup other than a worn chain, and I have no plans to change the friction shifter on an old bike.

OLD SETUP
6-speed Sunrace Freewheel (straight-cut, 14-28); 6-speed chain (don't know the type).

NEW SETUP
7-speed DNP Epoch freewheel (beveled or ramped cut cogs, 11,13,15,18,21,24,32); KMC Z50 chain (1/2" x 3/32", 6-7 speed). The KMC chain box says "Indexed."

BOTH SETUPS USE:
Long-cage rear derailleur (Shimano Exage), triple chainring (28,38,48; Shimano Alivio, 18-24 months), and friction shifters.

THE REASON for the change:
chain wear and skipping, wanting two faster cogs and a lower hill-climber cog. The new chain was adjusted for length using the chain adjustment equation on Parktools web site. After installing the chain, I doubled-checked and eliminated any chain kinking. The rear derailleur is adjusted and shifting well with the new setup. When I re-installed the rear wheel, I placed a thin washer between the right-side axle nut and the dropout for chain clearance on the 11-cog but it might not be necessary.

THE PROBLEM:
The new chain is intermittently skipping in a "skip-forward" or half-shift" motion; less so when riding casually, more so when riding under load; less so on the 48-tooth chainring, appears to be more so on the smaller chain rings.

What's wrong with my new setup?

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Assuming you're getting the friction shifter properly adjusted, most likely your chain is too long. –  Daniel R Hicks Dec 30 '13 at 11:54

2 Answers 2

sounds like your chainrings are worn. Look at the rings and see if they are "shark fin" shaped rather than symmetrical - a google image search for "worn out chainring" will show examples including some really, really worn out rings. The old worn out chain likely wore out the rings as well.

I've had exactly what you're describing on a couple bikes and it was remedied with a new chainring up front.

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In 18-24 months, seems unlikely that the chainrings are worn out. –  Batman Dec 30 '13 at 23:55
1  
I disagree. I've worn out plenty of inexpensive rings in that amount of time. –  OCary Dec 31 '13 at 0:43

Two thoughts, maybe with the washer the wheel is ever so slightly out of dish, meaning your chain-line may not be as straight as you would like it. The fix for this would be lose the washer.
The other may be a problem with your friction shifter holding it's spot over a longer chain. Was your chain the correct length before? It sounds like it likely is now but if it was to long before less tension would have less pull on the shifter. Is the shifter tight?

The chain ring items is surely worth looking at, see if worn out or bent. I haven't answered anything on this before so I may not be able to get back to you as I may never find this website again :).

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