From what I remember is that the Dart 1 is an elastomer and spring suspension fork and pretty much the bottom of the range for forks. The Dart 1 didn't have a lockout only pre-load adjuster.
Your options are limited as it's not really worth rebuilding this type of fork, but if you're mechanically inclined you could dismantle it to see if there's an obvious problem. Just watch your eyes when the springs shoot out and impale your eyeball. If you're just riding the bike on the road you could switch it out for a cheap rigid fork, or if you want to retain the bounce of your current fork the Dart 3 is quite cheap.