Yeah, it makes sense that it would be an electrical failure...if it were a jetting issue you would know very soon after the bike was started, it doesn't take _4 miles_ for the bike to warm up to running temperature.
The one thing that would become faulty after such a long time of operation would be something like a condenser, or your coil, or??
Whatever it is, if it runs fine with an 86 jet for the first mile, it'll run the same for any other mile with that jet.