Something we noticed a few years ago while replacing some clutch plates on our KLX140 - the plates would be extremely dry even after just running the engine.  Taking a closer look at the inner hub - there are no oiling holes like there are on a typical MX bike.  We added a few holes to our inner hub using a 2mm drill bit, and installed new clutch plates. We have seen improvement in overall clutch life and performance modifying the inner hub with a few holes.  Something to considering trying the next time you replace the clutch in your KLX140.