What causes "dieseling"

AJ/FormS said
most of those emission carbs have what is commonly called, a "throttle-kicker"; which is like an electric fast-idle device.

Dale Davies said
An idle solenoid can fix it

This kind of add-on is just what I need on one of my carbs on a 2.2 with run on issues. Does someone make a retro fit piece that closes the carb tight when you shut off the switch?