Put in the B7ES plugs. They will help as they are hotter than what's in there now. You do not necessarily have any other problems. Take a look at the spark plug 1978 spec change in the link (last bullet for 1978 model). It seems that different plugs were needed under different riding conditions. (Kawasaki also changed the plug recommendation for the KZ650's after the initial introduction)
Specs
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
