got a new throttle cable at the local shop, the guy is always happy I come in to buy something because I'm the only person in town who buys his xs400 crap
got a nice discount too
still more expensive than buying online though, but at least I know I got the right thing and I have it straight away
Then I went and stripped a phillips head on the throttle grip housing, sawed a new slot in it and got it out, put the new cable in and put it all back together.
Tried to start and it wouldn't fire up. Diagnosed it yet again with no fuel delivery to the carbs, checked my tank for vacuum which it seemed to create, and cleaned off my tank epoxy liner (which I put in last week) from the inside of the fuel filler cap which seemed to help. Tip for future epoxy liner users: stick some tape or something on the inside of the fuel filler cap before applying liner.
Finally got the fuel filler cap to 'breathe', and it would still not run nicely. Just fire up a bit, stutter and die. Turns out my inline fuel filter was clogged up again...
Replaced that and off it went.
Went for a test ride and the bike was acting crazy, the clutch was really inconsistent (sometimes engaging half pulled, sometimes still disengaged without pulling). Also the throttle seemed jumpy. After checking all the throttle stuff (thinking: that's the only thing I touched so the only thing that could be wrong) I notice I jammed the clutch cable between the carbs and the engine.
Got it unstuck and all was well.