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[Andy C]

RocketRR

I finished early today to had a look at the switch.

Yes, it was assembled dry, yes there was some crud on the contacts, as I had no grease to hand I just cleaned the contacts and re assembled, I'll get some silicone grease, strip and re assemble.

[Noodles]

Water in the tank? Check your fuel cap seal.

[RocketRR]

Well done Andy C this is a real problem and should be checked. As in the other post I found out about this mid roundabout while leaned over .... scary beyond words.

[vince]

Hi, as it happened on a roundabout ie turning , it could be a break in the loom around the headstock. As you turn, the wire separates . I had this on a bsa b40 . Another problem was cutting out at traffic lights which turned out to be a shorting out brake switch. Vince

[Andy C]

Noodles - hopefully not water in the tank as the bike is kept in a garage and I have not been out in the wet for ages.

Vince - I'll give the loom an inspection around the headstock.

[RocketRR]

Mine happened in a straight line too and since cleaning and greasing the switch a year ago it has not done it since.

I know these are all good suggestions but this really is the problem and as Andy found there were dirty build up clumps on the contact which acts as an isolator.

I cannot emphasis this enough Himmy owners check your switch as it will show this arcing build up.

[johnan1383]

I have read about this happening a number of times, and there are some videos on Youtube about it.

The cause is the ignition switch and wiring.
Sometimes the wires become detached from where they are soldered to the bottom of the switch, or they fray and eventually break due to the constant movement of the steering.
Also, as said, the contacts in the switch itself can become contaminated and need to be cleaned.
Some people have replaced the existing wire for better quality, less likely to fray, wire and extended the length to try to relieve a bit of the stress at the switch because of the movement of the steering. And obviously made a better job of the soldering.

I have no idea if the switch and wiring have been improved over the years but most of the examples I have heard of are in India, so perhaps early bikes? It is a very dodgy thing to go wrong in any motor vehicle, especially a vulnerable motorbike.