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.