Mule missing on back cylinder
 

2005 Kawasaki mule; fuel get to motor but misses on back cylinder, cleaned spark plugs and even change the position of spark plugs to oposit cylinder, still misses on back cylinder, suts up plug. Turn motor without plug in andplugfires, Any ideas what could be wrong. I have owned mule for three years and only have 30 hours on it, 20 of the hours are time spent working on the thing trying to make it run. This problem all started with mule dieing all the time and finally found that the metal gass tank was rusted badly. Am using plastic boat motor tank from Wal Mart for fuel tank now. All lines are clean and clear all the way to the carb.