After running my Yanmar 27hp for a while, there's a little liquid residue on the floor of the engine housing. It's black, watery and has a petroleum smell. It's not engine oil as I check that religiously. My novice guess is it's some semi-burned fuel mixed with raw cooling water. It's never more than what a paper-towel square will clean up. Any experience with this? Serious? What should be checked to diagnose better? Thanks!