on mine when i had a YF, it was intended to make the throttle close slowly, as rust collector was saying, but in my experience it would hold the throttle open enough that it idled at 1500rpm, so i took it off. mine was the model without a vac connection.
grantD is mistaken, the dashpot is entirely external and only actuates the throttle from outside.
if you want one, (its implied you dont have one), im sure i could ship it (or the whole carb, which has flooding problems) to you. if yes, PM or email me
--josh