carb adapter plates.....?

[fordman]

if it isn't the return spring i can say i have heard of the butterflys ont he throttle shaft being slightly out of position. that can lead to a slow closing of the throttle shft. or it could be the throttle shaft just has soem gummy stuff inside of it. if it wasnt taken off or the carb soaked overnight.

[TrikeKid]

Vacuum leaks can also cause a hanging idle.