I mentioned several months ago in this and the Ironman paint jobs thread that it felt like my '89 Ironman Expert had excessive drivetrain tension that was costing me power. The Suntour GPX rear derailleur spring tension felt excessive, particularly in the pulley cage pivot.

I bought an older style Shimano 600 RD a few months ago and finally got around to cleaning, lubing and installing it Friday. I set the pulley cage pivot spring to the lower tension retaining hole. Big improvement. Especially in the big chain ring and smaller freewheel cogs.

While it's only one ride so far, a familiar 22 mile roller coaster route, it was enough to confirm my hunch that the rear derailleur had been the problem. I'm looking forward to trying again Sunday with fresher legs. I rode a hard 30something miles Thursday, shooting for some KOMs (didn't get any, but did snag a 2nd place behind a friend's KOM, and several new PRs -- and that was with the Suntour GPX RD that was too tight). So my legs were kinda stiff and sore Friday. But without even trying hard I was coming close to my better times on that same route.

To sum up, the Suntour GPX RD had way too much spring tension in the pulley cage pivot. There's no secondary position, so I'll need to drill one. Probably an easier solution than trying to find a lower tension spring. Yeah, if I had a shop I could make a new square section coiled spring, but that requires heat treating. I've done it with flat springs years ago in my old rural home even with a fairly crude shop. But now I'm in a city apartment, no space for that stuff.

The Shimano 600 RD has some interesting characteristics. I'm guessing it was originally for a 5- or 6-speed bike -- I had to back the high limit screw way out and fiddle a lot with cable tension to get reliable shifting into the smallest freewheel cog. I'll need to use a bit of thread locker to ensure that screw doesn't back out and get lost, although it still has plenty of contact inside the derailleur. It just looks odd backed out so far. Fortunately the low limit screw was set perfectly, no need to adjust it.

There's a lot less movement than the Suntour GPX RD, and so far I've used the Shimano 600 RD only in friction mode. I just assumed it won't be compatible with the Suntour GPX indexing. But I'd already switched to friction shifting a week ago just for kicks, and now I prefer it.

I've also noticed there's far less drivetrain noise with the Shimano 600 RD. Just a wild guess but the GPX inner width between the pulley cage arms may be narrower than the Shimano 600. It felt like the KMC Z72 chain was constantly clattering against the GPX RD. But it's smooth and quiet with the Shimano 600. I may try a different chain with the GPX RD after modifying the spring tension.