I recently bought a 1990 FJ1200 and found the clutch would slip when I pile on the power - usually around 4000 revs...seems to be a common problem. My local service man reckoned a new clutch was needed. I did some online research and decided to replace the clutch spring with an EDC spring which should be slightly stronger than stock. As I don't know the oil change history I decided to do that as well.
In short, clutch problem cured. I can't say for certain it was all the spring, but the combination of new spring and new oil worked 100%.
The spring was delivered quickly, well packaged and certainly seems to be good quality.