Use a spanner/wrench to very subtly turn the eccentric spacer. This will tune the distance between wheel and rail.

You want to find a sweet spot where your carriage is not wobbly and the motion is perfectly smooth, but where there isn't excessive force pressing the wheels against the rail, as this will increase wear on the wheels.

When your carriage is tuned, you should still be able to turn each wheel with your finger without having to apply force.

Try to twist your carriage. If you find some wiggle room, your carriage is not properly tuned.

After tuning is completle, give one final tightening on the Hex Nuts.