Well to be honest I don't know much about the Z400 itself, but have adjusted valves on a couple different engines. You will have to take off the head cover. Then loosen the lock nut on the valves then adjust them to spec, tightening the push rod onto the feeler gauge. I do not know the specific clearance, but all this should be done at TDC on the compression stroke, while the engine is cold. And NEVER over-tighten!
Now with that said it is more likely that the valves would tighten rather than loosen, sense the valve faces and seats wear quicker than the cam and rockers. I would probably lean toward the cam chain tensionor or the chain itself. I know the 400EX had trouble with these. Adjusting the valves is always a good idea, you can just do it every oil change to ensure peak performance.
I will see if I can get the specs with a posting in the Z400 forum part, if you haven't done so already. I hope I helped a bit, but hope I can soon help more...
MY TOYS
406X Honda
Powroll stroker w/ Vesrah shorty rod
ProFab big bore
White Brothers cam
ported & polished head w/ +1 intake
Kibble White HD springs
38mm Keihin-choke removed
full DG RCM exhaust- want X-6
DID silent HD cam chain
K&N filter-No lid
02 Raptor-
High comp 720 kit
Web cam 461/496
K&N filter
Supertrapp exhaust
lowered 1.5"
92 250X-12:1 WIESCO .020/No-Toil filter
03 CBR954 - full 2brothers exhaust
95 Kawasaki Ninja ZX6 streetfighter
93 Honda XR80R
83 Z50R -K&N filter
98 Arctic Cat Cougar 550 liquid
86 Ski-Doo Citation