What's that gray/white powder inside intake port #3?

Well, big news at the SecaMaverick household... my estranged wife decided to give me back the XJ550 after a year of her not being able to keep up on it. I bought it for her eight years ago, and put a ton of elbow grease into it; we both rode it, and then I watched it drive away when she moved out and into her apartment last year. (Don't get me wrong; I mourned the loss of the wife, too).

Now she's seen fit to give the bike back in non-running condition, but I'm grateful. Still, there are several problems that have developed from her lack of mechanic skills. Here's the problem that has me a little concerned: There's a gray/white powder that's coating the inside of one of the rubber intake boots.

I removed the carbs because she had flooded the bike trying to start it one day (I'm told), and caught the left pod filter on fire somehow. Now one of the carb slides is stuck (black carbon is visible from the fire), and I'll have to give 'em a good cleaning again. Both the throttle linkage and the choke lever are seized.

I'm assuming that the gray powderish stuff in the one intake boot is some traces of soot that made its way through either during or after the fire... but I think I'll have to wait until I have the carbs apart before I can say for sure. At this point I'm just wondering how to go about cleaning the intake ports adequately before I get her running again.

I'd appreciate your thoughts or experiences with this, and I'll let you know what I find inside carbs 3 and 4.