He's blown at least three plugs in a new engine. It runs fine at the start, than it want's to quit later on. My first guess would be the tank is too low (raise the tank or rotate the how the engine is mounted or install a uni-flow fuel system). My second guess is a pin hole in the clunk line that gets uncovered as it runs low or a stuck clunk issue. Lastly, a restriction/kink in the muffler pressure line or fitting.

Oh, one more, Foaming fuel, isolate the tank with foam rubber.