(Not) such a bright spark!

There are two golden rules with this sort of electricity-both being broken here hence 'smoke'

First rule follow the basic colour code. In Norton case this is Positive earth hence all earth wires are RED. The 'live' is BROWN/BLUE. (as this is a MKIII there is also a large supply cable, possible black as well as the large RED earth cable) The other rule (that MKIII has a bit of trouble with) is ONE cable ONLY to the battery. In fact for most of our (pre MKIII) Nortons you should have one RED from the positive to earth and one BROWN/BLUE to the fuse.

In this case the owners have doubled up, and abused colours=SMOKE.

I know this does not help Mike Sullivan, but over this mode you cannot see where the non standard wires go, so someone with decent knowledge needs to sort this out.You certainly can't have an earth wire (RED) and a fuse wire to the same terminal 

.... my 1952 ES2 is negative earth as are lots of 1950s bikes.....

I actually prefer negative earth and converted my 850 Commando (rewiring appropriately of course) as an ageing mind and occasional lack of attention appreciate having consistency across my vehicles.

I also use a fuse in the earth lead as well as the live. I accept this adds a possible weak point but is does avoid the scenario when a screwdriver etc accidentally touches the live side of the battery and earth and fries all the earth wires in the loom......

You can have lots of fun and games when you start modifying your electrical system.  When I was preparing to put my bike back on the road, I re-made the wires between fuse and battery, so that the brake lamp was also fused.  I got a whole lot of mystery fuse blows, actually caused by an intermittent short in the brake lamp wire, which did not previously affect the fuse....

A little later, I also switched from positive earth to negative earth.  That meant switching the battery around, and re-flashing the dynamo.  I should also have reversed the ammeter, but haven't got round to that yet.  But clearly I am a heathen, as I have not replaced the wires with ones of a more suitable colour.  It's still brown for feed wires.

Paul

As per Paul above, changing the polarity of earth is fine with a mechanical regulator-which doesn't concern itself with polarity-BUT with any electronic regulator (which I supply) this would be taboo. BROWN for the live feed wires is acceptable, but BROWN is really-stop lamp to stop switch. BROWN/BLUE is the norm for battery live for Commandos (and Triumphs) but some times BROWN/WHITE for all post 1963 bikes. But it helps if the EARTH lead is appropriate-RED or BLACK.

Ian-having a fuse in the EARTH is acceptable provided it is in SERIES with the battery and the bikes earth. And in fact Triumphs in one of their guises had the one fuse in the earth. Yes it works and I cannot see any reason why it shouldn't But having been brought up with mains electricity (as well as bike electricity) having a Neutral fuse is actually taboo and very dangerous. So I still prefer the fuse in the battery live only. If of course you want to add seperate fuses for say headlights and ignition, a headlight fault that blows the fuse doesn't stop the bike. Be aware of the purpose of the fuse, it won't protect the item that has failed-often too late-but it will stop all that excessive energy getting out of the battery and causing 'smoke' in the rest of the bike.

Finally in the start up to this 'story' the nice blade fuse fitted to Mike Sullivans bike has RED cables, as RED is earth........you are signing your own disaster. Don't do it!

Have fun.

Re-settable fuses- sorry to disappoint you but but I have had very little to do with such things. The theory is fine but one wonders if the break current will further aggravate the blown circuit, in fact I doubt it, but it might damage the 'fuse' itself, in which case it might not protect as well in future. But is this an issue? The real reason for the fuse on our motorcycles is to stop the heavy power in the battery getting into the wiring and causing a fire. Just about most electronics will be long blown asunder well before the fuse blows. We normally have just the one fuse on our bikes, the snag with that is a blown fuse due to the head lamp will leave you without an ignition. So if you think you might be in for some 'fuseable' moments on the lighting ccts then perhaps a separate ignition fuse would be a good idea.