Engine Craftsman model 917.288563 tractor started backfiring while mowing

The tractor is 5 years old and well maintained. Cleaned the plugs and air cleaner recently. Fuel filter was replaced the end of last cutting season. This backfire problem started in the middle of cutting the lawn yesterday. The engine seems to run smoothly, with plenty of power. Put fresh gas in the fuel tank. Any thoughts? Thanks.

Not sure if this is still an issue for you, but I will tell you that I recently changed plugs and tightened it a bit too much. When I started the mower, it started backfiring. I believe the piston hit the plug and narrowed the gap. I took it back out, and re-gapped. Has run fine since then.