The start switch closes a pair of low current contacts in the solenoid. When that low current circuit is energized, it pulls a high current pair of contacts together, connecting the battery to the + post of your starter. This is the "click" you hear.
Some things to check:
Good clean connection between the frame and the green ground wire of your harness. I think on the DOHC bikes, it's under the seat on the right side.
Clean connections at the solenoid. The less resistance at these connections, the better the transfer of power will be. The solenoid can be disassembled and the contacts cleaned, but it may be better to get a new one (if one's available...should be). You should also check the Start switch contacts. They should be clean as well.
Starter in good shape. It should spin very freely once you apply power to it. Check the bearings and brushes inside the starter. I checked mine by removing it from the bike, and hooking it up to a NON-RUNNING car battery.
Good healthy battery. The battery is only good for maybe 18 amps DC at its best. If the battery is even a little bit low, it'll make for hard starting.
DO NOT USE A RUNNING CAR TO JUMPSTART A MOTORCYCLE. The current delivered by most cars is well over the 30 amps that the bike can handle.
That's all I can think of for now. Let us know how it goes.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.