I have a 2008 450 Sportsman, I have just purchased it from a friend been sitting in the garage for two years! I rebuilt the carb and installed a new battery touched the key a fired up. we rode the bike for a couple of hours that day and put a tank of gas through it. rode it off and on for a couple more days and last week I was pulling a small utility trailer moving some things around a yard, the bike was siting idle and made a noise like a backfire and the bike went dead, it has not started since. since then I have changed the spark plug, fuel pump, and cleaned everything I can think to clean. I am getting gas, spark, and air but it still will not start. Any suggestions on where to start next?