I agree with Dave. The right side of the carburetor feeds cylinders 1,4,6,7. If the idle passage on the right side is partially blocked you will notice it more at low speed. One thing you can try is to remove the idle mixture screw on the right side and spray Gumout or any carburetor cleaner into the hole using the long plastic tip. I had constant problems with the idle passages on my Autolite 4100 carburetor getting clogged. After I changed to an Edelbrock carburetor the problem went away. I assume you adjusted the idle mixture screws on your rebuilt carb after you got it back.