I have a JS28 John Deere its always ran perfect 1st pull. All of a sudden it won't even try to start. Has new spark plug...tested it, it is getting spark. cleaned out the carburetor meticulously today. Air filter is clean. Gas tank is clean and so is fuel line. Gas is brand new. When I have the air box on the engine won't make a noise when I try to start it. With the air box open it will back fire out the carburetor several times. I'm not sure what could make a perfectly working mower go from working perfect then 15 mins when I try to restart it its 100% dead. Any suggestions would be appreciated.