try a BR9-EYA plug instead of ES plug. did same thing with my 96 ZRT 600. tried new plugs, have gone about 4 times and no problems yet. i also unplugged all my carb switches. could be a problem is cold weather freezes carb open, so just watch that. i will sometimes put some isopropyl alcohol in my gas tank on really cold days, helps to keep carbs free
good luck..happy riding