Hmm. Ran into a similar problem with a friend's CB550 a couple of weeks ago. Was EVERYthing dead? No lights, no indicators, no horn, no click on the starter, nothing? And it started working shortly afterward without changing or doing anything? Could be a temporary loss of ground for your system (possible negative battery cable to chassis). Try tapping on the frame where connections are made (with the key on) and see if it wakes up. The next thing to check would be around the ignition key. Possibly it's dirty inside, and contacts aren't closing. Check the connector from the ignition keyswitch into the harness. In the case of the CB550, it was the main ground for the wiring harness to the frame. By jiggling the harness, we found the problem, fixed it, and got it running shortly thereafter.
Check out the wiring diagram in the factory manual, and start tracing down stuff with a voltmeter. You can either start at the battery end, and trace forward, or pick something like the headlight or oil pressure light, and track back from there.
Advice: use a oil-less or greaseless contact cleaner when dealing with switches. Oil just draws more dirt into the switch. Spray a small amount on your hand and check it after it dries. If it's oily, don't use it on switches. The best stuff I've seen is called Cramolin and comes in a spray can. A little goes a very long way. It cleans connections, and lays down a protective layer that resists corrosion. These 30 YO bikes are getting old enough that corrosion will start to set in, regardless of how weatherproof the connections are.
I hope this helps. Let us know how it turns out.
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.
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