84 rear end sags 3 inches under weight of bike

my 84 700 rear end would stay down when you sat on it. i put the max pressure in it and still didnt do much. i found out it has the 1000cc shock in the back and it has a 4 setting preload adjuster. i clicked it over to 4 and after i sit on the bike i can still pick it up 3 or 4 inches in the rear. it rides pretty firm. i have checked the little control arm the shock bolts too and it swings freely.

 

is there any type of shock i can use to switch to a spring style rear suspension instead of the air shock?

 

Do a search on this topic and you will find lots of info. You'll prolly end up talking to Jamie D.(he's on this forum). He mods newer shocks for the Gen 1s.