I have a john deere f525 with the 48" deck and i have not been able to get my mower to start since i ran it out of gas last week. When cranking on it the mower will have a light puffing sound and the occasional loud backfire but refuses to turn over. I have tried changing the spark plug with no luck. Also i tried dumping the gas out and filling with fresh gas. Any help would be greatly appreciated due to the fact i have not got the money for a new mower at this time and my lawn is starting to look pretty bad thanks.