With the key in the RUN position, the brown wire will show the coil+ voltage back up through the firewall and to the ignition switch; this is because that brown wire is not connected to anything by the ignition switch in RUN and the coil - voltage backfeeds back to the switch terminal.
But your reading of only 8.7 volts on the brown wire and at the coil + is a possible sign of an issue. It should be close to battery voltage which will drop some when cranking but normally not that much.
2. Please do the test that I asked a couple of days ago so we can move forward some and start sorting out what is good and bad. Place the spark wire from the coil 1/4" from metal and remove the wire from coil -, then put the key in RUN, and then use a jumper or screwdriver to ground and unground coil - . See if you get a spark each time you do this. Don't be holding the jumper or screwdriver blade when you do this or you will get zapped. This will tell us more of what is good or bad in the system.
I did that test and when the ground is made the arch is there in the power wire from the coil when its about 1/4 from a ground
3. Test the coil resistance please. Do this with the coil - wire disconnected.coil resistance is 1.3 ohms
BTW, the irregular arcing is not normal and is the symptom of not running.
