Remove the throttle cable from the adjustment bracket when you're trying to make sure the throttle is fully closed.
Then verify the throttle plate screw isn't keeping the throttle from fully closing.
Then reinstall the cable to the cable bracket, and adjust it so that the cable is not pulling on the throttle, should be just a very slight amount of slack.
In this photo it's the small set screw and jam nut, just to the left of the inner cable pulley. This shouldn't be touching that throttle mechanism tab at idle/off throttle, the part below the screw in the photo. Otherwise it's not fully closing.
