chances are the jetting is just to rich.find out what your main jet is and let us know.i own and race kx's and if jetted properly i never ever foul p[lugs even when just leisurely trail riding .he might have a sticky float thats allowing it to get to much fuel when he shuts it off and the fuel flows out the carb right into the cranks case then it loads up and fouls upon restart.
chances are though thats it's just the jetting.
i recommend a 155 on the main jet needle in the middle position,48 pilot ans the air screw 2 1/2 turns out from bottom.thats for a solid running 98 kx250.

 

Good advice, Killerkx327, he did say his bike was modded. Someone could have dumped anything in it for a main jet.