'99 WR400F Fouling Plugs

I just picked up a '99 WR400F last week. I took it to northern Michigan this past weekend to go riding, but it kept fouling plugs. Does anyone know what might cause this?

The bike has a White Brothers E-exhaust (full system). Is there a spark plug that is better to run than the NGK that is recommended? Could it be because I was only running 91 Octane gas?

Please help. I went through 4 plugs in two days. :)

if your carb's jetting is stock, it sucks and accelerator pump sucks too. I would start here.

Pull the carb apart, and RECORD all your jetting specs [main jet, pilot jet, needle jet & clip position, main air jet, pilot air jet, air cut valve still installed(?)]

After you get all that, post it here and we can move forward! :)

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