KDX's are well known spooge factories, especially if you are running the overly rich stock jetting.
You should see some improvement by leaning the jetting out, but there may be a nice reservoir of goo in your silencer.
A reduction in spooge might be had by running race gas, but relatively few KDX riders choose that path.
To some extent, a trail bike running on pump gas and being ridden at moderate speeds will spooge. It should clean off pretty easily with a rag and some WD40.