Check the battery voltage while you turn on the ignition and while trying to start. If the voltage drops significantly, the battery is weak. I had a battery in my ZN700 that checked 12.6 volts but died when I hit start. That said, there are several devices in the start circuit that may not be making good contact. If the battery checks out, get a wiring diagram and start troubleshooting.
1985 Kawasaki ZN700 Shaft, 4cyl
1989 Yamaha Venture Royale
2007 Yamaha Royal Star Venture