Sorry for no answers yet. I haven't heard from my son yet as to what has been done. We usually talk every Sunday, but missed this week.

I'll post as soon as some information comes in.

He was going to manually close the throttle, and if that didn't do the trick he was going to spray some WD40 around the carb mounting rubber and crankcase seals if he could reach them.

 

Not sure if the older bikes have reeds but that is the exact same issues I was having on a newer kx250. Sand will blast the tips of the reeds and let in to much air causing the bike to rev high. Good luck!