Idle only affects so much of a carb's overall performance. I'd be running down to be sure the timing is doing what you want before I blame the carb
If it's a used carb, no telling what someone did with the main jets and metering rods.
And, how big is it? might be a little large for a 318 if it's just a stocker, and the AFB is one of the large ones What is the carb no? Right on the front, stamped on one (left?) mounting ear.