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Check the clamps around the carbs. Climbing RPMs, and RPMs that fall slowly when the throttle is closed, is typically a sign of a vacuum leak.
After snugging the clamps, you can spray some carb cleaner around the boots at idle to see if you hear a change in RPMs - which would mean there's still a leak.
Have you checked the air filter? If not, just confirm it's clean and in-place.
Are the caps removed from the idle mixture screws on the bottom of the carbs (hopefully)? If so, check their adjustment. They should be close to 2 1/2 turns out. If it hasn't been checked recently I would make sure the carbs are synced before digging in too deep.
Check that the throttle cable has the appropriate slack at the grip, and manually rotate the cam on the carb (where the cables run to) backwards firmly when it's high-idling to see if it changes.
Run 87 octane without ethanol, or the lowest grade you can get without ethanol. That will also effect the mixture, making it richer than gas with ethanol. Do all of your tuning with fresh gas of the same type you will be using consistently.
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