If the rear shock is bouncey, it needs rebuilt or replaced, tighting the spring wont help at all. Take the fork cap all the way off. You can probly use a crescent wrench if you dont have the right socket or wrench to fit it. I would first drain the oil from the bottom of the fork, then add the fresh oil in the top. If it leaks as bad as you said it does, it will likely leak out pretty fast, and the suspension action wont work like it was intended to.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes