Hi gang, I have a 78 kz650b with a problem that I can't seem to track down. Years ago the wires on the horn came loose and shorted to the exhaust. This killed the bike. After fixing the fuse that was blown not sure which one is was now. At any rate since then the headlight appears to be dimmer than it should be. Everything that I can think of has been checked and or tested to ensure its working correctly. New battery through the years no difference, new voltage regulator,different bulbs for the headlight all are dim.if you hook the headlight straight to the battery it is bright like it should be. The only thing I can come up with is a voltage drop through the ignition switch of between 1 - 2 volts that goes to the headlight circuit. Any other ideas. All the grounds are tested fine.

I would verify you have a Very Good Ground between battery/frame and the front end assembly. If it is bad, then circuits on the front end(Horn, Headlights, front blinkers) will try to get a ground through the fork bearings and can pit or burn the bearings and races.
Temporarily jump a wire from Battery negative to headlight Negative and try to determine it is a ground problem or a hot problem.

HI OK justed rechecked the ground connection going to the headlight no change at all in the brightness of the headlight. If I go to the hot side from the battery the light brightens up. Just took ignition switch apart to clean up contacts they looked OK not burned.measuring the voltage at the headlight I get 9.35 when the headlight is on. When its unhooked its 10.35. Battery is fully charged evrything else seems to work OK its just the headlight circuit as far as I can tell. Checked the dimmer switch and the battery voltage is the same as at the headlight. Can't find any burned or broken wires removed the wiring loom from the bike and checked the connections can't find anything. Running out of ideas:(

Measure from ground to Battery, in and out of fuse holder, then Ignition in and out, headlight high and low, most I see you have done. That is the basic path. May be a relay(bad contacts) to reduce load through ignition switch.
You could take a hot wire from battery(20 amp fuse would be a good idea) then touch at each of the above points and it should reveal the problem.
Battery voltage seems low.
What is voltage when running?

OK will try not sure its going to do any good as I think I have tried to trace out all of the obvious routes. I will have to check the voltage when running pretty sure its OK because I have changed out the voltage regulator before and it didn't change the problem. I had the stator checked by the dealer and its good they found no problems waste of time.on the ignition switch on the white wires its battery voltage only the red wires is below voltage.

OK I'm back here's what I found so far. It looks like a poor connection at the fuses for one.
The coils when hooked up drop the voltage by 2 volts going to look into the coils to see what's up with that.ignition switch checks out OK same with the headlight switch.:rolleyes:

The coils have a primary and secondary resistance, if you don't have Service Manual, compare the readings with each other.
If you take a jumper from battery positive and touch to one side of the fuse , then the other side of fuse holder may reveal bad fuse holder.

OK checked the coils per the manual everything is within spec. still don't know why the voltage drops by at least 1-2 volts. Doesn't make sense. And yes the fuse holder is bad will replace with some ATO fuse holders and fuses. Applying battery voltage to the coils doesn't change the voltage drop so I really don't know what to do next. Working on the carbs now so when I get it all squared away will check running voltages.:(