My z400 will not crank when it is cold.  I have to roll it off.  It is getting sufficient fuel and spark.  I have recently changed the timing chain, but have had no luck.  Any responses would be appreciative.  

have you checked the compression?

sometimes if the compression is lower than it should be, even a couple pounds lower, it will become harder to start.  
also the carburetor could be too lean or too rich.

try taking the airbox lid off and try to start it cold.
this could help you determine if the fuel/air mixture is off.

you could also check the fuel tank for water, and remove the bowl from the bottom of the carburetor and inspect it for dirt or water.

as well the magneto and stator could have some rust or dirt or both built up from riding in wet conditions.
this will cause it to have a weaker than usual spark and be hard to start.