IthaDan 

Early 90s shimano had a tension screw- I know for a fact that 735 XT and the corresponding LX RDs had a little cam with a flathead screwdriver slot that you could rotate around to several different index points to set preload on the spring. (halfway down linked image, on the right) Pretty sure at least one generation of tricolor had this as well. I have almost no experience with XTR or DA of the era to say definitively either way. So I won't.

Unfortunately, in a transition to "light action" shifting (or, according to some, as an effort to calm the rising surge of gripshift converts by reducing the RDs capability to keep the cable taut, rendering gripshift unreliable at its job), shimano changed their parallelogram springs to coils from corner to corner and greatly reduced the return tension. This meant that contemporary rear derailleurs (the 737 era of XT) were incompatible with gripshift.

This created a space in the market for aftermarket hacks to get the two systems to play nice. There were two schools for compatibility: reduce friction in the cable or increase the return spring. It was then when we first encoutered the $40 per set GoreTex cables and housings, and, more bizarrely, the era of the gripshift bassworm. The bassworm was a piece of surgical tubing that would sit in the chainstay cable stop and provide supplementary return tension for the cable as the shifter released it. I guess it also kept the rearmost length of housing from getting clogged up, but the primary purpose was for spring assistance.

Bassworm:







That's kind of a long way to go to suggest you look into something like a gripshift bassworm to assist a weak return spring, but I was on a roll.