In that case it's entirely possible that you installed the eccentric incorrectly. Look at catalog photos, or even photos of a real Berkshire to make sure the eccentric is properly oriented. They are usually "keyed" with tabs, etc., so there are only two possible positions for it. The wrong one will be obvious as soon as the wheels begin to turn.
Next, look for something else binding in the running gear. Check for side rods that are bent. Make sure all of the hex bolts that attach the driving rods are snug, etc. Also check for the little brake shoe details rubbing on the wheels, especially the rear-most wheel with rubber tires.
You can also troubleshoot this way: Remove the shell. Put the chassis up on blocks and gently turn the flywheel with your fingers. Keep turning it until you feel it tighten up (bind). Then wiggle all of the side rods to see what's binding.
The worst case scenario is that you have a driving wheel "out of quarter" but this is rare. It's probably something simple, and you'll find it if you look hard enough!