I have an older craftsman mower with a briggs 12.5 hp mottor it had a replaced key for the key way ran grate then wouldn't start key way still looks good but the thing just spitts fire back out the carb any ideas

You need to pull the flywheel back off and make sure the key has not sheared in half. Sometimes it can look whole but it is not. If it is backfiring then I bet the key is not right. Why did the key need to be replaced anyway? Did you hit a root or something big to stop the motor cold? If not,then something internally went south.