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kdxrider.net a few KDX groups on FB too.
big guess here... the powervalves are gummed up ( they do need cleaning every now and then if sputtering stalls/ low RPM) or the water pump needs attention.
if he does break into the carb might be a good idea wot change the mid body gasket which is not part of the normal carb rebuild kit. for that you need to go to a specific vendor ...
you might have 2 issues vs just one impacting this.