Speaking genericly, the purge system should not be funcioning (purging) at idle. I worked on tons of cars of all makes and models w carb and purge systems, they all either connect the purge to ported vacuum or have some creation somewhere so it doesn't purge at idle. The amount of fuel vapors coming from the canister can vary greatly depending on how long car has ran, how hot it was last time parked etc etc. This varying amount of fuel/air can really mess with idle mixture. So they don't purge at idle, and start to purge as you open throttle. Off idle/cruising the little bit of fuel/air from purge will not make much diff.
If you can hook line to a ported vacuum source you will be better off.
And if it is sucking directly from tank or bowl w engine running you are wasting a lot of fuel.
Again, this is the way all cars---that I have seen-- work. They use diff devices to accomplish this. Nex time I see a stock 79ish fiat I will take a look and see how they do it.
