with a gap between the set stop and the throttle arm ?

Sometimes an engine will require you to run a larger idle gap then what is needed to have a near instant idle down.... Some weather conditions can cause that, extra drag on the engines idle like a clutch bearing....sometimes a fresh plug... a different heat range plug....basically many factors can cause an engine to carry RPM after a hard pass... Some days it seems engines are more prone to it then others....and many times what is causing it today is not the same as the last time LOL........ even stale fuel will require you to run a larger idle gap, as if you don't the engine will flame out.... A old glowplug can cause that as well.......