She runs! ...but backfires

I can't even begin to explain how a) utterly surprised and b) completely exhilarated I am that I got my XJ650 running, and took my first spin around the block tonight. A couple of things bugging me, though, and hoping I could get some insight (I know this has probably been answered before somewhere, so apologies in advance to Fitz, Rick, and the other veterans who have helped me out so far, for the redundant question):

1) I turned the petcock to Prime to get the fuel flowing, and after about 10 minutes, it began leaking out through the drain hose that runs from the airbox. Is that alone a sign that I have a float valve/needle issue? Is it more likely that it's a leaking needle seat, or that my float level needs to be adjusted?

2) The bike also backfires, but more so at higher revs than lower ones. After reading a couple of threads, it seems like this could either be what's left of the old fuel burning out, or possibly that the plugs need to be changed out. (I added some new fuel to the tank, but there was still some old fuel in there).

Does the combination of these two issues indicate a float valve issue? I just replaced two of them, and the other two seemed fine (and I know the common wisdom is to replace all four valves at once), so just curious if I need to run the bike for a while to clean out the old stuff, or if I need to pull the carbs again?

As always, appreciate the help!