Be sure that you are using the correct lever-shock oil (available from Moss and others).
If it hasn't been changed in a long time, I'd cycle the shock up and down with the side
cover off (I think it has about 6 ot 8 screws holding it on...not sure), and get all the old
oil out of there. You may also wish to remove
the large top nut (where the shock valve is located) and clean that out too.
A lot of folks use a hypodermic-type pump (or even a turky-baster) to re-fill the shocks.
Be sure you are filling it through the correct fill-hole (basically just a small screw you remove
....I'm unsure of the fill-hole location on that shock).
All told, if you fully drain the shock, it may take a 1/2 pint or so. New oil mades a huge
improvement on our street Sprite (our racer has tube-shocks).