Well let me start by saying good luck with fixing the bike. I found some info from another site that might be of use...
https://goldwingdocs.com/forum/viewtopic.php?f=6&t=63458&p=368295&hilit=backfire#p368295
https://goldwingdocs.com/forum/viewtopic.php?p=194239#p194239
The summation of what...
Having been around small aircraft most of my life, water in fuel is always around (in varying degrees). Pre-flight checks always included draining a sample of fuel from the tanks to look and see if water was excessive. I've seen tanks of fuel pumped out and new put in, because of this...