Since the recommended tire pressure on my 81 650 seems kinda low, I choose a pressure between the owner's manual & the tire mfg. then adjust if necessary after riding.
If the problem isn't the tires. when you rebuilt the forks, did you reference the 85 ZX550 Kawasaki factory service manual? There should be a fork spring service limit & fork oil specs. Some say the stock springs weren't that great to begin with & they often times go with a different weight of fork oil.
1981 KZ650-CSR. Competition, Sports & Road.