So when you move the X carriage left to right, with no belts attached to the pulleys, you're getting binding?
Loosen the eccentric nuts and see if the motion continues to bind with them loose.
Did you make sure, when you installed them, you have them in the "open" position, where the bolt is as far from the slide as possible? Ie. the hole in the nut is as far away from the slide as you can position it?