Thanks for the suggestions.
After doing a bit of internet research I found that BTH mags were used a lot on what are now vintage British motorbikes. One site for Vincent enthusiasts gave me a lead when it said that if the grounding brush was gummed up then the 'kill switch' would have no effect.
I ...

Thanks for the suggestions.
I'm definitely on the correct mag. I was checking the continuity with a multimeter, which seemed to show that the switch was connecting to ground ok, but then had the bright idea that perhaps the switch could pass a minute current (ie the multimeter) but may not pass a ...

Can any of you engineers out there help me with a baffling problem.
A fault has developed which is stopping the left BTH mag (non-impulse) from grounding. The only way to kill the engine is to turn off the fuel.
I have checked what I think are the obvious things - mag cap making good contact with ...