Thanks for the replies guys. I have to admit I'm not surprised by the lack of replies. It seems people either don't do anything internally with their forks, or a few intrepid people replace them with more modern ones. Other than chopping the springs to lower the bike, modifying the stock forks doesn't seem to happen.
Anyways, I've been brainstorming options that will require research, but may be interesting if it all works. That is a big IF! But here goes:
First option - Replace forks with late production XS650 forks, brakes, etc. Then rebuild/upgrade the forks with cartridge emulators, springs, and possibly longer tubes.
Second option - Replace the XS400 fork tubes with longer ones. The later ('77-'84) XS650 used 35mm x 23" long fork tubes, the same diameter that the XS400 Maxim and Seca used, I don't know the XS400 tube length yet.
Third option - Replace the forks with 36 or 37mm forks from another bike. The triple's would have to be machined to fit the larger tubes, but there is enough material there so this shouldn't be an issue. Front wheel and brake would also have to be changed. The replacement forks would likely need emulators and springs. For example, the Kawasaki KL250 Super Sherpa has 36mm forks with 230mm (9.1") of travel and uses a 21" spoke wheel.
Points of interest:
- MikesXS offers XS650 tubes that are 2", 4", or 6" longer than the stock 23". The fork travel isn't changed by this length increase, but the longer tubes would raise the front and provide more ground clearance for the engine, which I'd like. Rough measurements suggest the 2" longer tubes may be adequate (assuming the XS400 tubes are the same 23" length as the XS650), but that has yet to be decided. I'd also have to buy top caps and other bits as the XS650 uses different parts. I don't yet know if the fork internals are interchangeable.
- MikesXS offers knock-off cartridge emulators for the XS650 at 1/3rd the price of genuine RaceTech emulators. If they work well, this will help stay within the budget.
- MikesXS offers progressive XS650 springs, but they are almost 8cm (3 1/4") shorter than the stock XS400 springs. Not sure what the deal is with that yet.
- Better brake disks are available for the XS650, if required. Doubtful, but nice to have options in the future.