Originally Posted by
awrightus
Looking at the guts to this thing, and I'm sure I'm not using the right terms here, but there's sort of a sprocket and then a ratchet that the thumb shifter uses to move the sprocket. That piece isn't even touching the sprocket, it just moves right over it so there's just "full play" when I shift, no parts touching.
That's the common failure mode. The little "pawl" gets glued to it's pivot by the dried out grease. You need something stronger than WD40 to dissolve out the old grease. That's why I mentioned alcohol, and
bradtx listed mineral spirits. The teeth on the ratchet wheel can also get caked full of enough grease that the pawl might skip over them.
You need to approach the issue like a dentist, cleaning each tooth on the ratchet, and working the pawls back and forth until they move freely. The springs on the pawls are quite weak.
Originally Posted by
awrightus
Interestingly, I now have two shifts on the left, which is an improvement,
If the guts of a MC33 left shifter are the same as a MC30 shifter, then there's a dried grease failure mode that gives you just two gears. Getting the third (last) gear requires freeing up a stuck pawl.
Originally Posted by
awrightus
So, seems like the best option would be to buy a replacement. I'm poking around online for "shimano 3x7 shifters". If anyone has specific recommendations, model #'s, web sites, etc, I'd sure appreciate it. Don't need high end, just functionality.
You don't have many choices. Your situation is compounded by having the brake levers integrated with the shifters. If you can find some separate 3x7 shifters, then you'll need to find some brake levers too. You'll need ones for "canti" brakes, which are themselves getting hard to find. Or you can butcher your wonderful looking STX-RC units and cut off the shifter mounts. As for integrated units, I think your only choice these days is the canti brake version of the Shimano ST-EF50 (EasyFire) shifters. Don't get the V-brake version!
The picture you posted shows a nice condition bike. I think the only thing wrong is that your grease is dried out. (And maybe the cross threaded bolt...)
BTW, if you give up on the STX-RC units, put them up on eBay. Even "broken", they might sell for more than new EF50's.