Single coil

L.A.B. said:

Yes, or connect white/purple directly to white/yellow so bypassing the white/blue.

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here is something I found in response to a similar answer by Grant Tiller
Agreed Grant. But the ballast resistor lives next to the coils and the link wire (white/purple) is very short. Certainly doesn't extend back to the ignition switch. So yes, the ballast resistor should connect to the coils, bike isn't going to run otherwise, but nowhere else unless there's an electric start.

But you raise a good point, OP should check that coils are 6v and use the ballast resistor if they are. But if the coils have been replaced with 12V then power (from kill switch) goes directly to the coils and the ballast and white/purple link wire can be removed.

wavelenght2 said:

But if the coils have been replaced with 12V then power (from kill switch) goes directly to the coils and the ballast and white/purple link wire can be removed.

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Yes, you can bypass the white/purple and use white/blue to the coil or connect white/yellow to the coil.

Fast Eddie ask why anybody would want to stick with mechanical points?
This is not a compelling reason, but perhaps worth mentioning.
A points system uses a mechanical switch: it uses no current to trigger the ignition. If you have enough juice to fire the coils, the bike will run. Pretty sure you can't say that about an electronic system.

L.A.B. said:

Yes, you can bypass the white/purple and use white/blue to the coil or connect white/yellow to the coil.

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excellent

Stephen Hill said:

Fast Eddie ask why anybody would want to stick with mechanical points?
This is not a compelling reason, but perhaps worth mentioning.
A points system uses a mechanical switch: it uses no current to trigger the ignition. If you have enough juice to fire the coils, the bike will run. Pretty sure you can't say that about an electronic system.

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I wasn’t asking that per se…

I do believe that EI is superior in general however because points wear and move out of adjustment. Points are therefore excellent things, that work well, but only when kept clean and correctly adjusted.

There is also a potential issue over the quality of modern parts.

But… my question was why use points AND a modern single coil? That’s the bit I don’t get. If using points that fire each cylinder in isolation, I’d use a coil for each set of points, just like standard. If using EI that runs a wasted spark, I’d use a single coil.

I’m not saying it’s the end of the world, just don’t see any real benefit, but I do see hassle and potential for errors. Just seems to be being different for the sake of being different to my mind,

Das ist alas.

Stephen Hill said:

A points system uses a mechanical switch: it uses no current to trigger the ignition. If you have enough juice to fire the coils, the bike will run. Pretty sure you can't say that about an electronic system.

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Pretty sure I can

The electronics uses a few milliamps for control. The output transistor causes a voltage "loss" of about 0.2v max. That's the extent of what you say about triggering.

Points, if closed, in perfect condition, and clean will have a zero-voltage loss. However as soon as you start running, you're charging and discharging the condensers and you're wearing the points, and the voltage loss is greater. Points almost closed and slightly opened are wasting current (sparking) - transistors have no such problem and therefore do not need condensers. To make it worse, on a Norton, you're throwing away approximately 1/2 of the voltage via the ballast resistor which means a huge overall current loss in the system. On top of that, the auto-advance, works well when it does and is terrible when not.

Most electronic ignitions are highly superior to points for these reasons. The Tri-Spark (and some others) also provides idle-stabilization, an advance curve very close to the standard auto-advance, and nothing to work on ever unless you have to remove the timing cover. Tri-Spark will fire down to 8 volts - the coil or coil pair is getting 8v too. A points system at 8v is supplying each coil approximately 4v and there's a good reason why the world left 6v coils behind long ago!

Finally, to have a hope of reliably starting a MK3 with anything other and a perfect, full-charged battery, the built-in ballast resistor bypass is used with points - there's no need with an EI.