Is it from the recycled crankcase air? I had this problem from a Honda I had. the recycled crankcase gasses were greasing up my IM and gumming up my PCV valve. I fixed it by removing the hose that carries the gas from the valve cover to the IM and replaced it with a small breather filter.
But as far as the fuel, its a really good question. I have no idea how fuel would work its way up instead of following vacuum down into the cylinders.