Some throttle plates have a hole drilled in them to allow some air to pass even with the throttle closed all the way. If you're sure the blade is aligned properly and there are no vacuum leaks, try plugging the hole (if it's there) with a pop rivet. That will restrict the airflow so you'll need to open the blade slightly with the idle speed screw.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!