At the bottom of the Carters on the side facing the valve cover, there is a little screw. To adjust my dual Carters, I turn the screw all the way in. Then I turned them out 1 1/2 turns each and started the truck. I turned in the screw until I reached rough idle then I turned it 1/4 back out. Remember, whatever you do to one carb, do to the other.
Do you have a uni-syn tool? If not, I HIGHLY suggest getting one off eBay for cheap money, your truck will thank you for it!