All GPZ900s to 1986, the 85 & 86 GPZ1000RX and 86 ZGs suffered from tuliping but Kawasaki identified the problem and replaced with harder valves for the 87 onwards.
The difference between 6,000 mile valve checks should be less than .001" and once the motor has been broken fully (around 20k miles) should stay the same from one check to the next, if you set them correctly.
Tuliping was where the head of the valve would wear quite rapidly allowing the valve to recede into the head, thus closing up the gap.
See the pic at the bottom of this page
http://cog-online.org/clubportal/clubstatic.cfm?clubID=1328&pubmenuoptID=30529