corkscrew

I'm replacing the cables on a early 90's Giant Rincon. It has a 21 speed Suntour drivetrain.

I have the cables all replaced, and the shiftings farly tuned. However when slowly shifting the rear gears, the chain has a tendency to sort of half ride the cogs, before eventually clunking into place. This tends not to happen when quickly shifting.

What could be causing this?

DRietz

Honestly, this depends on the quality of your components. I can't say for sure, but there may be face wear on either of your RD pivots. This can cause the derailleur to move into a shift position, but move a bit, causing uncertain shifting. If it shifts when you're riding it, I'd say go with it. If it ain't broke, don't fix it.

Also, it may have something to do with the tension on the cable. Try increasing the tension on the adjuster barrel on the derailleur itself.

HillRider

I expect your cable tension need to be refined.

Al1943

Have you measured your chain for stretch?

tellyho

I think both of the obvious culprits have been covered, and it may be a combination of both tension and chain wear. It sounds like your tension is just a little too high.

helicomatic

Don't forget cassette wear as well, sometimes the teeth get thin enough to sit between the inner and outer plate on a worn chain when shifted slowly.

DMF 

+1

My thought too. This can also happen if the chain and cogs have mis-matched wear. If the chain isn't badly worn now, was it ever?