mk3 commando starting problems

The feed to the ignition is only working when the solenoid is providing active power to the starter. The MK3 has an arrangement whereby 12V is fed direct to the 6V coils via a purple/white as the starter button is pressed for a stronger spark. After the button is unpressed it reverts to a yellow/white wire feeding a ballast resistor that drops the 12v to 6v so the coils get 6V.

So if the yellow/white wire has been disconnected/knocked while you were changing the assimilator or the ballast resistor has failed then you get the symptoms you describe.

Go to

https://granttiller.com/norton-commando-wiring-diagram

and scroll down to the MK3 diagram and check the yellow/white run which includes that stupid connector under the tank (another likely failure point). 

You have the kill switch on!

When you press the button is bypasses the kill switch and ballast resistor as mentioned, so it starts.  As soon as you release the button the engine will stop.

Guess why I disconnected the kill switch about 40 years ago.

Hi Cameron.

Please before you do anything, check the kill switch. The symptoms are spot on for that.

If it worked when you put it away, then that is the most likely problem.

Tony

Totally agree with the diagnoses already stated!
 

I also think your issue is with the off/run/off switch.

An easy way to confirm this is to temporarily take a wire from your battery negative terminal to the terminal on the ballast resistor where the WY (white/yellow) wire goes.

This will bypass the ignition switch and the handlebar off/run/off switch (as well as a few dodgy connectors along the way)

With this wire temporarily attached, start the bike and see if it stays running (I think it will)

…you now know for sure where to start looking!

If you have electronic ignition installed instead of points, it is likely that your ballast resistor has been disconnected, and the WP (white/purple) solenoid bypass wire has been removed too.

If that’s the case, run your bypass wire directly to the ‘hot’ negative on the electronic ignition black box (it’s usually the white wire on most of them)

Don’t be afraid to reach out if you have any questions.