If I slow kick mine through the compression stroke, it will backfire on the second or third kick. But it starts fine. I'm no expert, but you've got me stumped also. If I weren't down by Galveston, I'd like to see exactly what's happenin'. I know you said you only replaced the carb jet. There's no chance your timing chain is loose enough that it could have jumped a tooth is there?
what size pilot are you running? i assume you're uncorked... The only backfires I have ever had on any bike [real backfires, not carb fires] have been on running bikes that ran poorly. a fouled plug on my now-sold DRZ caused a lot of backfiring, but AFTER the engine was lit. I can never recall a true backfire flame-belcher like this on a non-starter.
I guess ANYTHING might happen at random but the most likely 'Occam's Razor' is that I inadvertently changed/moved/lost/misplaced something.
Something I never got an answer to, guys: is there a tiny O-ring or something inside the channel the Pilot screw screws into? anything that could have fallen out unnoticed?
tomorrow with luck i'll have a loaner carb