This has disappeared now but I'm curious as to what could have caused it. Now that both of our bikes have no issues for the first time in a few months we went out for a ride...

As the S40 had just been finished the gf gave it a wash, I don't know if it's relevant but the header is wrapped. After the wash the bike sat for a little while not too long. Then when we went out I was behind and noticed that it seemed to be, I say backfire, but that could just be fuel exploding in the header. Anyway it's the normal sort of occasional bang you get on the S40 except it was continuous and did not go away. After about 5kms it was getting worse.

Stopped and had a look and it would not idle just kept dying. I turned up the idle screw to make it idle and that seemed OK, then I had a blast down a side street, stopped and revved it up and came back, by the time I got back the idle speed was up so I turned it back down and after that it was fine.

The only thing I can think off was that the wet header pipe caused this, but then I can't see how that would make the idle drop down to nothing. So it's no big deal because it fixed itself but as I said, I'm curious as to what could have caused this. It was exploding quite a lot.

Just washed? Most probably a slug of water blocking a fuel jet in the carb that was able to clear itself. Or maybe some water in the electronics.

I don't know how you wash motorcycles, but I don't use the  high pressure coin op places or any kind of pressure washer. I also steer clear of spraying water directly on the carb(s), electronic modules and electrical connectors.

OK thanks for those suggestions.

@Batman, sounds like if it's possible to ground the plug from a bit of water on it then it could be that. I just want to understand this so we can take more care, not that we don't already.

We do take care to use very low pressure garden hose, a lot less pressure than you'd get from simply riding in the rain. The silver covers are on, but that doesn't mean a bit of water didn't get under that.

Which part of the electronics would be susceptible to this. btw this has never happened before which is why I thought maybe a completely wet exhaust wrap may be the culprit although I don't know why.

Due to the fact that the missing/ backfiring is intermittent I would think it has to do with plug wire, the other things people said would tend to make the bike die outright. If your coil 's output is 20,000 volts ,that doesn't mean the plug always uses it, if conditions are near perfect the plug may fire using at little as 8 or 10,000 volts. the conditions in the combustion chamber are always changing even at steady throttle . If the next cycle is less perfect it may demand that higher voltage, and if the wire to the plug is wet or has a small crack ,or both ,the voltage may find a path of least resistance and ground out on the casing rather than going through the plug. The bike slows, you move the throttle, the fuel ratio changes, the swirl changes, etc . Conditions are again better and the motor starts firing , until it happens again. That's my guess. If the wire is over 5 years old I'd inspect it .If not it may be just a one time fluke.

I have had similar experiences when I got some water in the carb bowl(s).  Not on this bike but others.  Water lays in the bottom of the bowl and intermittently gets sucked up into the carb.