Usually the carbon is in the form of a carbon impregnated thread or string. You can usually just strip the wire like the carbon center was a normal wire. Then fold a piece of the carbon string back along the insulation, and crimp a new plug connector onto it. Don't worry; the carbon wires are often called "resistance wires", but you won't cause any problems by shortening your wire.

As captain mentioned, some spark plug connectors have pins that pierce the insulation and make contact with the carbon string in the plug wire. If yours is like that, just remove the original plug connector, cut the wire as short as you want it, and then reinstall the connector.

AV8TOR