[DiamondJim]

It sounds to me as if your car is very rich at idle, your power valve may not be receiving enough vacume to keep it closed at idle. When you partially cover the center carb it is increasing the vacume signal to the center carb and closing the power valve(it revs because it was so rich it is almost flooded, closing the valve corrects this problem). How much vacume(in inches) do you have at idle? You may need to cut on few turns off the spring or better yet buy one of the spring kits that Jon Hargrove sells(The Carb Shop). -Jim