Did it have fuel stabilizer in it while it was sitting for a year?
Even if it did, my first suspicion would be that the carb is fouled.
But usually that doesn't lead to backfiring, just not starting. On
the other hand, the backfiring did start when you turned off the
gas. If it was timing related, a valve, etc, curious that it started
just when fuel was shut off. I suppose the float could be stuck open
flooding it badly.

After checking the spark plug condition and for spark, I'd pull the
carb and rebuild it. If you get a service manual it should be fairly
easy. By taking it apart, you can completely clean it and remove any
gunk. The rebuild kits are cheap, look on Ebay. I got one for a
Tecumseh engine for ~$12.