Turns out the problem wasn't electrical but a fueling issue that's now sorted. However, I thought I'd share my experience for anyone considering a coil conversion on an older bike.
The first consideration was the primary resistance as I was replacing two three ohm coils and stick coils are typically under 1.5 ohms each. Then there is the actual size of the coil and whether there is room under the tank as they sit higher than the standard plug caps.
I found some that were the right size (albeit with an o ring added to make them fit snugly in the plug well) but they were just under 1.5 ohms resistance. Wiring two in series I measured each of the circuits at 2.5 ohms which was not far short of the 2.6 and 2.7 ohms of the existing coils and within the standard spec of 2-4 ohms.
I then had to make up a wiring harness which was not too much trouble using heat shrink tubing and waterproof connectors. Once the old coils and HT leads were out there was plenty of room to fit everything. The first couple of times she seemed a little reluctant on start up but I think it just behaves differently on the choke and there are no issues now I've got used to it.
I've now done a couple of hunded miles and will admit that I can detect NO performance advantage over standard. The only benefits I can see are fewer connections to develop problems, particularly on the HT side, a much tidier area under the tank and the possibility that if a coil does pack up I can probably limp home on three better than I would on two cylinders. Price wise it came in slightly cheaper (under £100) than replacing the standard coils/HT leads/plug caps with good quality replacements (and I still have the option to fit the originals back in).
Would I do it again? Yes on price advantage alone unless you're going for originality...
