Hey,
i have this same problem with my 1976 Yamaha XS360. once my bike warms up, the revs will shoot up to about 4000rpms and just stay there until i kill it.
i used brake cleaner spray to locate an air leak in the "linkage" that you mentioned. spraying break cleaner on the spring made the bike drop from 4000rpm back down to a smooth 1200. but then in a few seconds, its back up to 4000. also, like you said, if i turn the linkage counterclockwise, the revs come back down to 1200.
when i found the air leak with the break cleaner, i thought all i needed to do was buy new butterfly throttle shaft seals from Mikes XS, install them, and I'd be good to go. but if my problem is being caused by bad o-rings, why would turning the linkage counterclockwise drop the rpms back down to 1200?
i took the carbs off the bike tonight and i can see a lttle more light coming in from the left butterfly than the right one. and then if i turn the linkage counterclockwise, i can hardly see any light coming through at all.
can i assume that my problem is that the butterflies are just not closing tight enough? maybe they're letting enough air sneak through to push the revs up to 4000?
what do you guys think?