Thought I'd chime in here; over the holidays I had a friend of mine over and we had a run on the bikes. I let him ride my 748 and he had mentioned that it had backfired while he was riding it.
Since then, its backfired on start up twice, both times starting fine. I notice a distinct smell, like gas mixed with a metallic sent. Would this be the same issue you guys are talking about?
Other than that, she runs fine.
2000 996 SPS
2002 748 (just about back together!)
2003 Honda hornet 600 (SOLD)
1969 BSA Royal Star 650 (Being re-resurrected!)