Cycle Garden offers a complete range of US and Italian made cables for most early v-twins. Replacement cables in stock include: throttle, choke, brake and clutch.
Most brake and clutch cables cost around $30-35 new, so there isn't much point in working over an hour to re-lube and free-up an old cable when you're better off replacing it.
When cables are working well and properly lubed, the interior sliding cable is free of resistance. If at idle you have a noticeable drag on any of your Guzzi cables, expect that tension to be multiplied under load. We've had great luck with a compound called "Grease Monkey" as our cable lubricant because it adheres to the cable and doesn't evaporate.
The right cable adjustments can do a world of good for your Guzzi. Remember that clutch cables should have a bit of "free play" in order for the clutch springs to exert the maximum amount of pressure on the plates.
Another tip for cable adjustments is to set your clutch to fully disengaged well before the clutch lever hits the handgrip. This is done by turning the thumb-screw cable adjustment located where the clutch cable meets the fulcrum at the handlebar.
So for Guzzi cable needs, give us a ring.