Bike running, main fuse and tail light fuse read 12v.
At both ends of the fuses?
Front light fluctuates between 2.5 and 5
That's a pretty cool meter. I've not seen one like it before.
Anyway, it is time to begin noting where you are placing the probes along with the readings you see. Details like these can be important.
For the above readings, where are you placing the black probe? The meter just tells you what it sees between the probes. It doesn't know about the circuit. That knowledge is the domain of the one wielding the meter.
The 75 CB550F routes headlight power thusly:
Power routes from battery to Key switch. Red to Black (distribution buss).
The Starter button switch routes Black to Black/Red when the starter button is NOT depressed.
Black/Red routes to the Headlight fuse.
Fuse output is Black/Yellow which routes to the left bar control H/L switch.
The H/L switch routes Black/Yellow power onto either Dark Blue or White wires to the headlight.
If your headlight won't light up, then you have a break in the above path or the green wire to the headlight does not connect to the battery NEG terminal as it should.
You might check the integrity of the main harness ground connection to the frame at the Coil mount. There are, of course, other green path connections to the battery NEG terminal, and those are just as important as routing power from the Battery POS terminal to the device using the power.
Cheers,
