One nice piece of insurance I used to use on my XR-400. I ran the crankcase ventilation pipe into the top of the air box, but in the middle of the line, I installed a small fuel filter. At one time I had also run it up along the frame and under the tank, and had it sticking behind the head light. But you could always smell the oil fumes that way. Try the fuel filter. It works.

Honda Forever,
Gary B.