It is called a 'backfire' and often means the timing is way off or an intake valve is not closing (could be a bad valve spring, valve seat, slipped cam timing,etc.).

Today I adjusted the valve clearance to the factory setting and the tractor worked fine.

How did the valve adjustment get off?

The two-cylinder Briggs engines with overhead valves are well known for needing periodic and rather frequent valve adjustment.

Sounds like it is time to add a jamb nut on the valve stud.