I had this problem one time before,...turned out to be a piece of gasket mat'l from the carb baseplate that got stuck on the SECONDARY throttle blade holding it open ever so slightly, just a tinny, tiny, winnie little bit! that the idle would never go down no matter how far you backed the idle screw off or OUT!...take the carb off and check to make sure the shaft blades are sealing front and rear completely.....