Backfire when getting on the throttle?

[671M900]

So I rode the monster to work today, and she felt off, most importantly, she couldn't return to idle, she would just keep dying! When I got home, I checked the heat of the header pipes with some spray water, vertical cylinder was only warm, and the horizontal would vaporize the water. When I managed to get home, I found a loose battery post screw, made it nice and tight and idles fine now.

However, once I get on the throttle, BAM! Backfire from the cans, everytime. Any ideas? Some inadequte fueling due to one hot cylinder and one fairly warm? I haven't ridden her for like 2 weeks, so it could be old gas... But, too much carbon (seafoam?)? Sticking valve? Needs a valve adjustment? The last valve adjustment was 1000 miles ago, and it's a 99 900 with FCR41's.

[64duc]

  Check your plugs and wires. The one that was misfiring may have fouled.