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Hi currently looking for the cause of no spark on the rear cylinder and going through FAQ and previous posts.
Secondary resistance with the +/- tolerance comes to 3300-4070 both mine are 3470 ohm so all good.
Primary with tolerance is 0.31-0.37 (2 significant figures) after subtracting the resistance of my leads I get 0.5 ohms. Should I be concerned? IAm assuming my none firing vertical is not related to these values- or are they!!! The spark plug was VERY reluctant to show me a spark when I pushed its side against the cylinder head nut. I had to find a very clean piece of metal to earth it. Probably unrelated but I’m mentioning it as it may be a clue I cannot see.
Keith MM in a VERY windy London.

You could swap the coils over and see if the fault goes with the coil. What is the voltage to the coils,these things arnt known for their high quality electrics and it's very possible you have lost enough volts from the battery to the coil(due to crap connections/joints and switches )that there is no longer sufficient juice to power the coils. Do you know how to do a volt drop test ? Your mention of earthing is a good one and one thing to do as a possible cure ,and regular maintainable is to keep said earth points on the frame in good condition .

Great nudge in the right direction. I started to do some voltage drop tests and obviously started at the battery. The large + cable definatly had a worrying wiggle when gently fiddled with!!! When I took it out about 6 or more strands had fractured. Well that’s going to increase the resistance and eat into a bit of voltage. It then gets worse (or better if your fault finding) I could even pull out a couple of strands from the crimp area. iIRC a crimp should reduce the x sectional area to something like 95% of the original area. Clearly this crimp has never been properly since the day it was made!! I will make up a new cable this morning and report back. Have a hunch I have found the problem. Struggling to upload photo of dreadful crimp on phone- will try later on pc

Looks like I'm having a conversation with myself but this may help others . The rather hard polymer boot on the coil leads make lining up the over moulded connector difficult. A couple of times I thought I had made a good connection but when removing found the male terminals bent over!!! not good for continuity. I have cut them off and crimped new terminals.
I also had a bit of a pull on the positive lead mentioned previously when in a bench vice- well THAT was never crimped properly from new. Bloody well pulled out!! Did not go for a run after each fixed (that bloody body work!!!) but after both 'fixes' it now runs as it should so one or both of the fixes did the trick!! Also checked for any voltage drop after these two fixes and found very little drop through the ignition circuit. (The wiring mod FAQ post has a very good diagram)
Keith in windy London
ps all the broken strands in the photos were like that BEFORE I pulled it off. If it was a perfect crimp I would expect the crimp to hold and the strands to fracture.

Well done on sorting it out, it’s good to check and check again that battery terminal looked bad!

Pleased you got it sorted