Briggs & Stratton intek 18 HP engine contunuously backfires

I received this mower from my brother in law it would not crank. I found that it needed a new starter. Replaced the starter and it starts but only now the mower is constantly backfiring. Any thoughts on what to check now. I have replaced a fuel line, cleaned the carburetor, adjusted the valves to .004" and nothing seems to help. Should I change the muffler ? I just can't make sense of this issue at this time. The mower has not ran for three to four years.

I would check the flywheel key to see if it is sheared.

Look on the bottom of the carb and see if there is a plug for the cut off solenoid, if there is pull that off and try it. if these go bad it can cause a problem.