I have a 3 year old Craftsmen lawn tractor. Up until Saturday all was well. Saturday when I started the tractor, it starts fine but now backfires. The idol is also not steady ( goes up and down ). It does this at full choke and with the choke closed. I have changed the oil, filter, air filter and spark plugs. The Gas filter looks OK, so I have not changed it. It still has the original battery. Could the battery be the problem?

No. Chek for fuel contamination; and valves