1973 850 Commando - No spark

Did you connect the battery the 'right way' round. i.e the same as it was with the old battery. Or did you initially connect it the other way round?

Positive or negative earth does not matter as long as everything is the same way round.

Lights and horn do not care.

When you say 'regulator' is that the simple rectifier and Zener or something modern?

I do not know if the electronic ignition has reverse polarity protection (anyone know?).

If the only thing you have done is changed the battery and you did connect it the wrong way round initially, and now you have no spark. I think you know what the answer is, and it is expensive!

There is every chance that the Boyer ignition unit has been 'killed' by reverse polarity. I can test it but it is better to send it to Boyer Bransden. If it is only the amplifier box, that will have died, they are less than £100

Checking out wiring in this situation with a 'circuit tester' is virtually a waste of time. The bike CAN be negative earth. What you have to consider is....any electronic ignition and any battery charging has to be altered also LED lamps have to be corrected (LED flashers). Ignition coils will not be harmed and they will still work (reverse polarity) but the spark will not be as good. I personally would NOT do this without a complete rewire other wise you have a RED wire Earth and even on the battery in which case you end up in a Mucking Fuddle as per. Carry On.

More correctly 'Lucas' branded. The Lucas name is now owned by Wassel.

New looms are available either cloth bound or PVC tape bound. I'm not keen on the cloth bound variety, i find them far too stiff to mould around the frame and the cloth covering holds all the oil and shite that the engine expresses. That said, i prefer to make my looms as you can customise it to your exact requirements. For example, you can get rid of all the under tank sleeve connectors and put them in the headlamp shell. Neatens things up and reduces the amount of bullet sleeves used (less connectors = improved  reliability) Al Osbourn sells a kit to rewire bikes. That is if you can read and understand the wiring diagram in the manual!  

You don't have to understand the wiring diagram, they are all the SAME. If you follow the British Vehicle wiring standard from the 1960s and follow the colour code it is easy. The colour code comes with my wiring kit-see the web site A O Services.co.uk

If you just want the bike to start/run then of course you have to replace the Boyer amplifier and connect it with regard to CORRECT polarity with respect to the battery. initially you do NOT have to worry about any voltage regulator. Disconnect for the moment. decide if it is Positive or Negative Earth, connect the battery thus and the BB ignition then start the engine. The lights and horn will not care re polarity, but any LED lamps will The bike WAS positive earth it should have a RED wire throughout the loom.

If it were me I would not buy a new harness as a lot of the wires will be redundant as you have electronic ignition etc.

Make your own it is not difficult and there are good wiring diagrams on the net for modified neater setups. I prefer original bullet and spade connectors for Norton's and a good set of crimping tools and a soldering iron and your away.

It is also very rewarding and neater.

I have a 2A which has always started first kick until letting me down late last year. It got me home albeit not running perfectly. I found sediment coming from a decaying tank lining, so blamed that for the problem. Now I have sorted all of that out I find the bike does not want to start! I replaced the battery last week (Motobatt 9MU) and the bike started and ran for a few seconds before cutting out and nothing since then. I can get a spark at the plugs but not brilliant - but the nearest the bike comes to starting since is the occasional backfire after a kick. 

The bike has BB power box and BB electronic ignition. The carb is a single Mikuni and the whole lot has been trouble free for more than 20 years. Now it won't start and I am too old to kick the thing more than two or three times! I wonder whether one of the BB boxes has failed but have no idea how one might test them? 
 

There are two Boyer Brandeston units on your bike, Ignition and a regulator/rectifier to charge the battery. The later, if it has failed is NOT stopping the bike starting. The bikes correct starting and running relies on a charged battery, Charge or (recharge) the battery, check it with a meter must be up to12.8 volts ish. If below 11V do not attempt to start.
Checking the regulator/rectifier is a separate issue. There are details on my web site, I can check it as well but the problem here is getting the bike running and the BB reg/rect is NOT the issue.