My experience is a small diameter vent hose provides enough restriction that a metering valve is not necessary. One less part. I forget the size of my vent hose, something like 5/16" I think.
The tricky part is finding a place to discharge the spittle that is open to air. Usually a galley sink is the answer. Joe, your drawing has me concerned. The arrangement of discharging the spittle to a below the waterline thru hull works when the engine is running but will not allow air into the loop freely when a siphon tries to start (vacuum dynamic). That's the reason I mentioned the end of the vent hose needing an open air connection. Mine connects to the galley sink drain at a point above the waterline.
edit
Disclaimer: maybe "thru-hull" on the drawing doesn't mean what I thought it did.
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Neil
1977 Catalina 30
San Pedro, California
prior boats 1987 Westsail 32, 1970 Catalina 22
Had my hands in a few others
Last edited by ndutton; 05-06-2019 at 02:39 PM.
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