Mower starts then dies and backfires

[dnsmith2188]

Craftsman rear engine rider with 13.5 Briggs engine with new plug, air filter and carb. Starts right up, doesn't run real smooth, after 15-30 seconds it sounds like it clogs up then dies and loud backfire out of the carb. It will start back up no problem then does same thing. Guy that replaced carb said the valves looked fine. Any input where to start?

 

[Fish]

Sounds like it could be a loose valve seat {intake}.