Removed it once more and opened it up. The screw post that the battery cable connects to is soldered to the coil winding – or at least it should be. Mine had come adrift. I set to straightening the wire (it’s more like a bar) and cleaning it up ready to resolder. The post would solder nicely but no way would the solder stick to the winding. I cleaned and heated, set fire to most things near by but the coil wire/bar wouldn’t solder.
I was ready to resign and buy an exchange unit but fortunately it was past shop closing time so I cleaned it one last time and crimped the post hard to the wire/bar and when I fitted it I twisted the post to hopefully make best contact.
This seemed to work well at least for the rest of
The BMW thermostat was fitted in the block. I bought an M14x1.5 tap on eBay and made a wafer thin helicoil type adapter so it would fit.
I then set the ignition timing again, and reset the carburettors to the manuals spec and tried running the engine again.
Lots of smoke and it still idles for a bit, then roars up to 4000rpm and back to idle. It does tend to spit back through the carbs occasionally too.
Quite baffled.