Well sort of sounds like sheared flywheel key but can't imagine bogging down in heavy grass causing that.

Does your brother have a mower? Sounds like he lives close?

Walt Conner

Peach: Have replied to your duplicate thread this morning , try not to duplicate threads its is xtra work and potential for confusion for forum members . As Walt has stated I also felt that perhaps the flywheel key had been damaged (twisted or partially sheared) quite often tall wet grass will weaken an already damaged key to the extent that engine
may continue to run , but upon restarting attempt it will refuse to fire . Quite often a backfire situation will occur during running prior to the engine stalling . I have had a key so loose that the engine started ran but sounded like pre-ignition (ping) during load . Upon inspection the key had shift just slightly which caused the timing to be off enough for the symptom . Some of the new keys are quite light weight Sacrificial Lambs it does not take much to damage them .

I don't see "new spark plug" written anywhere here.

Good Point Tom !