Philbo

The frame is "Earth" or negative, if you want you could run a wire directly from the battery negative terminal, but I doubt it will make much difference.

I'm not clear from your post where the problem lies, is it the cylinder next to the front wheel that's not firing?

A good way to check is to put a working spark plug into the HT lead of the suspect coil, and either press the body of the spark plug against a clean part of the frame, or run some wire directly from the negative terminal of the battery and expose a good 6 inches of copper wire at the other end. Wrap this exposed copper wire tightly around the thread of the spark plug. (Be sure that the plug is nice and clean).

Making sure sure the plug is well away from the spark plug hole in the cylinder, turn over the engine. Did it fire? If so the, it's no the coil or the HT lead.

It could be the thread in the cylinder casing is too dirty to give a good connection to earth for the spark plug when fitted. Try cleaning it with a rag and a wee splash of petrol.

You could also run a wire from the negative battery terminal to a point on the engine, but if one cylinder is firing OK, this is unlikely to help.