I have a '99 FLHT (carb'd) and it has developed a nasty backfire (through the carbeureter) while starting the bike. Once I get the bike running it runs great, from idle to WOT and if I shut it off, it will start right back up if it's still warm but it really is hard to get her started. I'm guessing it may be a problem with the enrichment circuit in the carb but I would like to know if I'm missing something before I tear the carb apart.

 

I'd take a look at the pilot jet.
Scott