. My Greenfield Anniversary 12 reconditioning is all but complete, finally!
All I have to do now is sort out a spark problem....... I've fitted a new coil. All good. Motor won't fire. Fuel supply all good, but the spark plug's not igniting it. I can only get the spark plug to fire (out of the head) when I pull it slightly away from contact with the engine! A spark will jump from ground to plug, and the plug will fire nicely at the electrode. If I earth the plug to the engine/frame it won't fire!
I've never come across this sort of problem before and it'll no doubt be something dumb.
My only suggestion based on my own experience would be to try a new (new) plug.
Reading your symptoms reminded me that a vintage car bloke once told me that they would often leave an air gap in the HT lead (solid wire core, I guess) without it the thing just wouldn't fire! Was completely baffled by this when I heard it and haven't been given a decent explanation as to why this should be the case. I'd love it if someone could offer a good technical explanation!