My Specialty Spares act the same way. I had the a/f screws out 3.5 turns and had no popping. My plugs looked a little rich (although they say you really can't read plugs anymore with unleaded fuel) so I moved them into 3.25 and have a little popping. I kind of like it, me and Dinqua think alike!
I also get the loud backfire on start up if I touch the throttle. Mine is so loud it startles everyone and I'm thinking that can't exactly be good for the bike. This seems to occur whether or not the petcock was off. My mileage is also poorer than most, I never get more than 41 mpg (18 tooth sprocket). The bike runs like a champ except for that occasional backfire and low gas mileage.
