On the 5411 the antisiphon valve is an "elbow" of black plastic with the valve at the top. It is attached to two rubber hoses coming from the engine compartment and is located under the sink attached to the bulkhead. It is above the water line and keeps a warm engine as it cools from sucking in raw water from the exhaust into the engine. It is an essential part and should be cleaned periodically. Modified boats may have another configuration, this is the stock installation on early 80s boats with the 5411 engine. I suggest you google antisiphon valve and 5411 engine to get a clearer idea.