Before throwing sparks all over the place, I'd inspect all the connections, make sure they're clean and tight. When I got my '78, the starter didn't work at all, no click, no nothing. It turned out to be a very dirty and rusty cable connection down on the starter motor itself. That connection hangs under the bike so is subjected to lots of water and dirt kicked up off the road. It has a rubber protection boot but mine was split. I cleaned it all up really good, installed a new rubber boot, and it's worked fine ever since (nearly 20 years now).