No spark

Had a bit of a go sorting out a no spark issue recently & thought I'd pass on results as it may help others.
Bike is a 1973 750 Commando, mostly stock with Boyer.
Ran 10 miles to gas up & no start.
Diagnosed no spark, but easy fixes ruled out, so trucked home to garage.
Tank off, and found only 7 volts at Boyer lead connection. Battery at 12.9 V . Also 12.9 V at switch feed to harness and all lights are bright .
Original harness ( only 49 years old ) looking real tired, especially at connectors where the rubbers are severely cracked & almost disintegrating.
On my shelf for about 6 years is a brand new harness of which the installation got delayed many times. (Why hurry, when bike running good, eh?)
Installed new main harness. New soldered bullets and connectors where required.
Checked Boyer power connection with digital volt meter & it showed 12.8 V (Remember this for later)
Try to start bike, & still no spark.
Ran through regular coil & Boyer checks. Appears the Black Box was just not functioning.
Oh well, it is 30+ years old , so I went ahead and ordered a new Black Box. Coils were on fringe of resistance spec, and were older of some oriental source so I popped for 2 new Lucas ones.
Everything replaced.... nothing. WTF ??? Deep breath & think this through again.
Going over the electrical schematic & it jumped out at me. Head smack myself. Had not checked wiring to/from and at kill switch. (It works by breaking connection not grounding out)
Noticeable resistance showed on meter. Carefully disassembled switch. No green meanies, but contacts looked aged.
Dressed & cleaned contacts.
Bingo. Big fat beautiful spark... gave myself another head smack just because.
My mistakes & summary. I should have jumped direct power to Boyer. This would have eliminated coils & ignition components right away. Then, back trace and identify actual point of issue. Instead I condemmed harness without proof. Followed that up by trusting small digital volt meter 100%. Should have checked with test light. Little digital meter did not draw enough amps to cause voltage drop at kill switch contacts. Not something that is likely, but can happen and I know this.
Yes, the last thing I touched (The kill switch at the gas station) was the culprit. Yes I did initially check it & flick it to make sure it "felt" normal & wasn't stuck. However, always a great adage is to check on the last thing touched/replaced.
I don't regret replacing coils, but really did not need a spare Mk1 Boyer black box. (not a bad thing though).
Compassion & criticism welcome.

PS Darn cheap Norton electrical harness....it only lasted 49 years.... maybe 50 plus if I had of ID'd switch issue first & put off replacement a bit longer.
Cheers

My kill switch failed when the bike was only a few years old. I soldered a jumper across it and ran that way for over 40 years. Recently I decided that I wanted that function again, so removed the solder blob and cleaned the contacts. It failed on me again after about 1000 miles. Not sure what the long term fix is for that.

We've all done some head slapping, "DUH" stuff!

My favorite one is when I was trying to sort out why a Turbo Porsche wouldn't generate max boost at full throttle. After an hour of futzing around with a variety of settings/adjustments (car on dyno), I pulled the bunched up part of the floor mat out from under the accelerator pedal...full boost returned!

I have an ignition test rig. Ring terminal to install on battery neg, wire to a fuse holder w/15amp fuse, wire to rocker on/off switch that mounts to handle bars(cheap on ebay). Then wire with spade terminal to connect to white wire from ei box. Takes a few minutes to install. First thing I do when I suspect an ignition problem. Safer than plain jumper wire because it has the fuse and a means to turn the power on and off. Can even ride the bike with it in place.

I learned that those little jumper wires with alligator clips don't always pass 12V when trying to bypass the usual electrical system. I always make a good jumper wire with a ring terminal for the battery and the appropriate terminal on the other end. I spent a long time one day learning this lesson.

Been there done that and more....... Hind sight is always 20/20. Experience teaches us things that we wish we didn't know. Don't beat yourself up.
Cheers

Eliminated the Kill switch no joy feature a long time back. It's a challenge to keep them running reliably so why put another fly into the ointment ? Simply take the 2 wires and crimp/solder them together before the actual switch which you won't be using anymore anyways. This winter I will be heading off the issue before the pass by tracing it back into likely the headlight shell and fixing the 2 wires together there. One less problem to deal with Yah .

The kill switch on my bike is only there in case the throttle jams when it is wide open. The switch is on the handle bar and connected with plug- in connectors each side. If I have a problem with the switch, I simply by-pass it. If I was ever to drop the bike and it was lying there revving it's head off, I'd just grab the wires and yank them. The switch is one of these in which the contacts click closed when you press the button, and releases when you press it again. Its rubber cover is broken and water can get in and stuff things a bit. When I ride my bike, if the motor does not miss I never worry. If it misses, I fix it immediately. A miss can be due to a lot of things, but it is never good.
When I tune the jetting, I force the motor to miss, then I tune the miss out. If you do that with a two-stroke, you can cough-out a crank. Commando engines are more friendly.