95 425cc magnum carburater problems.plug is black when i run the machine hot, boggs down and looses power. carb settings are by the book,i even changed the old carb and put a newer carb in from a 2000 polaris quad. still fouling the plug,(using NGKBKR5E). The quad runs good when i put a new plug in,but when i'm ploughing or running it hot it continues to foul the plug(plug is black) Could it be something other than the carb? Any solutions?