Our 40 year old house has developed a smelly basement drain. We have sewer flies in there, too. We have poured lots of water down there, some bleach, and some insecticide to kill the flies (all at different times). The flies are now gone but the smell is still there. I've plugged it with a rag and covered it - smell is gone. I've read somewhere about a plugged vent stack - but I'm not sure what this is or how to look for it. Please Help.