since the carbs are a butterfly type(stock) unless they are sticking or the throttle is sticking, theoretically your sucking air somewhere. could be the boots from the carbs to the engine, could be the idle screw, could be the synchronizing screw between the carbs, hard to say.
usually an erratic idle, especially to your extreme, is caused from upsetting the fuel to air ratio, or leaning out as kc mentioned. lean meaning more air, less fuel.
hard to visually inspect the seals, ones behind the clutch and the other is behind the recoil. depending on the time lapse from your idle to wide open, if you have time, spray some carb cleaner behind the clutch and see if your revs increase prior to it doing it on its own. you can also spray around the recoil and the carb boots as well. even easier to spray the recoil side if you have an e-start machine.