fouling

Hey guys, I have a GSXL that's been causing me some trouble all summer. I've gone through quite a few troubleshooting steps so far and there hasn't been much of an improvement yet. So the issues I'm having are poor throttle response off idle and the thing fouls plugs in about 15-20 minutes...

I bought this last year with a trailer for $400, the owner said it had been fouling plugs, I replaced that battery and got it running. When it ran it ran like it was soppose to, powerful and didn't miss. Then one plug would foul and I'd start riding back home and the other would foul, eventually...