Check the upper pipe mount on the frame, and keep an eye on the chain rubbing into the swingarm. If it's the original kickstart lever, it will probably break soon and need to be replaced by a later-model kicker. I had all these problems with a near-virgin (under 10hrs) 1998 KX500 I bought last year.
I originally was stubborn and ran 32:1 like the book says. I was fouling plugs all the time, and had a bad bog unless I could hold it wide open for extended periods. Since switching to 50:1 Belray MC1, I haven't had a single fouled plug, and it runs much better.