Join Date: Oct 2006
Location: Green Bay, Wisconsin, USA
Make: GL1500 SE
At first glance I would think it is not PG's but if you think about it a little more it very well could be. It seems when they PG's fail they have a variety of symptoms. The one thing that seems to be common is the on again off again spark to the plugs. The OP did not mention checking spark when it was not starting. I wonder if the bike is n't cranking with no spark for a while. Loading the mufflers up with air fuel mixture. At some point the spark decides it is going to start and that is your loud backfire. Maybe the "loading up" is not loading up but just weak or no spark causing misfire. I would give the entire ignition system a good look over paying extra attention to PG's and leaking secondary voltage from plug wires, coils etc. I'm wondering if you have intermittent spark some how.
Worked on the "big rigs" for 45 years now just riding my Wing whenever I can. Gets cold in Wisconsin.