Could an air leak cause a slow return to idle?

 

Actually, a slow return to idle may be from several things. A very loose motor with thin oil doesn't have the internal friction tighter motors have. More likely on a newer motor is the idle mixture is off somewhat. As the motor settles into high vacum low flow operation, the mixture slowly gets rich and sloppy or lean and dry to the point the motor slows down over the course of a half minute or so. Many motors that have this issue, (and a huge percentage do,) eventually idle-stall dry or flooded(overcooled the spark-plug.) Properly tuned and matched carbs allow a motor to run at a very low idle indefinately.

 

yes
so could a rich mixture...to high of an idle adjustment