The old Guzzi forks have a sealed cartridge. Besides being marginal to begin with they are not adjustable using oil volume or shimming.
You can change the damping in stock Guzzi dampers somewhat. If they're the type that has a black plastic cap with schrader valve in it on top, the black plastic piece can be unscrewed, a rubber bladder plucked out, the damper drained and refilled with whatever weight of oil you see fit. If the bladder is still good, there a certain amount of adjustment afforded by pressurizing it. The bladder restricts oil flow through a series of holes at it's tip. More pressure = more restriction = stiffer damping.
On the original sealed dampers, it's possible to drill a small hole at the top, down in the M8-1.25 threaded hole where they attach to the top plug, drain out the original "fish oil" and refill with whatever weight of oil you see fit.
Not "adjustable" in the truest sense of the word, but can certainly be made to work better for little cost.
