Why is the lawn mower is backfiring?

because there is too much gas in the cylinder. to fix it, lower the choke. also timing could be off due to a run of thick grass or hitting object with blade. flywheel key could be bent just a hair. that is what it was built for because the key metal is softer if you hit something with the blade the key bend or shear which leaves the flywheel out of time with the coil.