This is my fifth season with my 84 Ninja 900.
I have been over the bike many times and have gotten to know this rig quite well.
The coolant pipes ( the bike has two, one behind the headers and the other above the carbs ) will occasionally leak a bit of coolant. If yours leak all the time then you will need to pull them off and replace all 4 o-rings that seal them to the block. This is a heck of a job as the fit is very tight. If you decide to do this repair you will need to pull the carbs as well. Make sure you get the exact same size o-rings for the pipe or you will be a 'cussing. When I did mine last year I used a bit of white grease to assist them while reinserting the coolant tube. If you choose to do both at the same time set aside 3/4 of a day. At the same time you may wish to flush the cooling system. I flushed mine last season and the difference was noticable right away. I used pre-mixed coolant from Honda and it works well. I also have a manual fan switch as the previous owner gave up trying to fix the original bits.
A few areas of concern on that bike BTW,
Fueltap, a must to rebuild.
Sidestand switch, make sure its clean and not interupting things.
A good site for only that bike:
http://www.gpz900r.co.uk/
MAke note of the parts section and the troubleshooting guide contained within one of the sites listed.