Firstly, welcome to the Forum.
new wiring harness
keeping + earth but switching to electronic ignition and regulator/rectifier.
You don't say which electronic ignition or regulator/rectifier you intend to use. If you haven't chosen, some general thoughts:-Regulator/rectifier
Choose one with specific DC+ (Red) and DC- (usually Black) wires.
Although the standard alternator on your bike is 'single phase' (2 wires, as standard White/Green and Green/Yellow), consider a 3-phase (3 AC input (usually Yellow) wires) reg./rec. Reasons I suggest this are: the standard alternator on your bike is feeble by today's standards and, when you ride your bike, you might decide you want to upgrade it; for the time being, a single-phase alternator can be connected to a 3-phase reg./rec. without any problem but, later on, a 3-phase alternator could not be connected to any single-phase reg./rec. you buy now.
Of the reg./rec. available today, I advise:-
. either 3-phase Podtronics
. or https://www.ebay.com/itm/12V-Voltag...250C-750-1000F-VTR-250-1000/382370502356
Any reg./rec., I always connect the DC wires directly to the battery
terminals (of the above, Red is DC+, Pod Black or Honda Green is DC-).
In the harness Brown/Blue wire (wiring diagram "NU") between battery
-ve and ignition switch terminal #1 are female blade terminals for the rectifier and Zener diode; these will need to be insulated securely.Electronic ignition
If you haven't chosen already, I'm guessing you'll choose from Boyer
-Bransden, Pazon, Tri-Spark 'Classic Twin' or Wassell/Vape; advantages and potential disadvantages to all.
If you haven't chosen ignition coils already, I recommend two separate '6V' coils similar to the original '12V' ... because you specifically mentioned originality in your first post and your bike has the standard mountings for them. Be aware that the '71 mounting allowed the bottom corners of some coils to contact another part of the bike when it was ... errr ... vibrating
... damaging the coil case;
later oif coil mountings angled the bottoms of the coils towards the sidepanels.
Unless you already have some familiarity with BritBike electrics, none of the fitting instructions are particularly well-written, nor the wiring diagrams particularly well-drawn:-
. in the case of B-B, Pazon and Wassell/Vape:-
.. their White wire should be connected to one of the White/Yellow (wiring diagram WY) wires connected to the ignition coils (wiring diagram #19); the other end of the White/Yellow wires is connected to the 'engine kill' push-button in the switch cluster on the right-hand handlebar;
.. if you use the harness Black/White and Black/Yellow (wiring diagram "BW" and "BY" respectively), you must remove the same-colours wires to the condensers (wiring diagram #21) or insulate the female blade terminals securely;
. in the case of Tri-Spark:-
Black/Yellow wire (not the BY wire in the harness) should be connected to one of the White/Yellow wires in the posted wiring diagram;
.. the harness Black/White and Black/Yellow wires aren't used;
. in all cases:-
.. the female blade terminal on the other, unused, harness White/Yellow wire must be insulated securely;
.. I never connect any e.i. Red wire just to some sundry bit of bike; like "desco", I extend it to the battery
.. I never connect the coils' series ground - the Red wire from the "+" terminal of what the fitting/installation instructions label "Coil 1" - to the frame or other sundry bit of bike; I always connect it either directly to battery
+ve or to the existing harness Red wires network.
the headlight, taillight and engine.
Not shown in any wiring diagram is the network of Red wires that were in any original Lucas
harness, should be in any reasonable-quality new harness and, as a general rule-of-thumb, provide the proper grounding for almost all electrical components:-
. Headlight - The original standard BPF (British Pre Focus) reflector/bulb bayonet cap should have a loop to take a standard bullet terminal on the end of a harness Red wire. If you bike's doesn't have the loop, be aware that the BPF bulb/lens/reflector is truly dire and you might want to consider a lens/reflector that takes a more-modern bulb with a standard H4 (3-blade) plug (which can be had to attach to a standard harness's bullet terminals). But you have to be careful which more-modern lens reflector you pick because many don't fit the '71 'flatback' headlamp shell ...
. Taillight - Lucas
seem to have started fitting them with a Red ground wire (in addition to standard Brown and Brown/Green wires (wiring diagram "N" and "NG" respectively)) from about mid-'71. If you're bike's doesn't have one, I either thread a Red wire into the unit beside the other wires, attaching the Red wire to the bulb holder, or attach it with a ring terminal to one of the mounting studs.
. Turn signals - The ground path is through the chrome over the plastic bodies, so not only does that have to be in good condition but corrosion and dirt that collect in the joints between bulb holder and body and body and stem affect the grounding.
Thereafter, that front turn signals screw into the headlamp shell is a good ground path, as long as the threads are clean and there's a Red wire from the harness attached to the shell; but the rear stems screwed into a painted tail-lamp mounting, that's mounted on a painted(?) fender, would be laughable if Triumph and BSA hadn't actually sold 'em like that ...
It's sometimes possible to squeeze another wire down the stem (it can be bare as it's a ground wire, but try not to poke it into the other wire's insulation ...
) - if I can, I solder it to a washer inside the bulb holder under the spring - or you might find it simpler to attach the ground wire to a ring terminal that fits over the stem and trap it between the stem securing nut and the tail-lamp mounting.
Finally here, I've already mentioned a number of mods. that must or should be made to any off-the-shelf harness to accommodate just electronic ignition and a regulator/rectifier. However, the original harnesses were built down to a (very low) price, most pattern harnesses mimic the shag-awful original specs., which can be improved with a few other cheap mods.; would you like me to post details?
Otoh ... modern wire in original colours is available, terminals and insulators that look like the originals are available, coverings that look like the originals are available; even buying the terminal crimping tools, the total shouldn't exceed the cost of an off-the-shelf harness; if you can build an engine that doesn't blow up, you should be able to build a harness that doesn't melt. I'm not sure about my ability to build engines that wouldn't blow up but I haven't had anything to do with an off-the-shelf harness in about thirty years.