Suntour index thumb shifters- rear shifts to larger cogs fine but not to smaller cogs
 

Old Suntour X-Press Accushift thumb shifters (7sp) and Suntour X-1 rear derailleur (7sp), Suntour 7sp freewheel, on an old Trek 930.

Bought this bike cheap at a garage sale, trying to get it working. I believe the above parts are all originals.

Problem: Shifting from smaller to larger cogs on the rear wheel works great, but going the other direction doesn't. The derailleur doesn't move quite far enough, so the chain just kind of rides in between cogs, and doesn't make the full jump to the smaller one. If I twist out the barrel adjuster any more, then the jumps from smaller to larger cogs are too big, and the chain starts to rub against the next highest cog.

(Chain definitely needs to be replaced, but I haven't done that yet. I don't see how that could be the problem, though.)

Any ideas?

Probably time for a thorough cleaning. Something's impeding the derailleur from taking up the cable slack.

Take off the rear mech, clean it, replace the cable, lube, and then adjust.
Remember to lube/oil the shifter too.

FYI
If your chain measures 12" from pin to pin you are ok. If it is more than 1/8" above 12" then think about replacing the chain and cassette.

Try operating the derailleur without using the shifter (pull the cable). If you can't move the chain to all of the cogs, then your problem is the derailleur.

The chain should be replaced when any one foot interval has stretched to 12 1/16 inches to avoid excess wear on the cogs.

This sounds like you turned the barrel adjuster the wrong way. Try clockwise.

Shifting performance will probably improve by replacing the shift cables and cable housings, especially the short housing found at the rear derailleur. Your problem could be too much friction along the rear shift cable.

+1 on replacing the cables and housings and lubricating all of the moving parts. Keep in mind that the return spring on the derailleur is the only thing moving the mechanism to the smaller cogs. Your shifter releases cable to make that shift, it does not "push" the derailleur over. Any friction in the system will cause sluggish upshifts. After you have gotten everything moving smoothly follow the directions from the Park Tools Web site. http://www.parktool.com/blog/repair-...nts-derailleur
Start at the beginning and do not skip any steps. Pay particular attention to their technique of moving the derailleur by pulling on the shift cable rather than using the shifters.

Thanks everyone - will try all these things. Also, for whatever reason, altering the B screw adjustment helped quite a bit. Shifting is jerkier now, but at least the chain generally makes it to the right cogs. Will give everything a good flushing out and re-oil, and see how it goes.

If you're using suntour black der. cable housing, they usually come with black plastic caps, and these crack and cause the very narrow housing to kink. I once found a source for stainless caps that fit in Japan and bought a bunch of them and over the years have been taking kinked housing, trying to unkink a little with toothless pliers, replacing the caps, reversing the direction and re-installing with liberal amounts of grease inside. The shifting (especially on the way to smaller cogs - higher gear) improved dramatically each time. Check both the cable segment from RD to chainstay, as well as from shifter to downtube braze-on stop. Both suffer the same kinking issue.

Just FYI, those aren't what most folks refer to as "thumbshifters", even though you do use your thumbs to operate them.