Don I believe this valve vented loop boils down to a small rubber “Duck Bill” The little arrow shaped end goes down in the vented loop and plastic threaded cap screws on top to hold it in Place. The little rubber piece can get bunked up and needs to be replaced from time to time. Your generator exhaust discharge should be above the water line and I believe the vented loop is a little safety measure as a generator on a sailboat under sail heeled over 15 deg under sail might be discharging underwater while running. Maybe I have my application confused I would call NL tech support for clarification I had a messed up vented loop on my holding tank years ago and it was causing my holding tank to fill up with sea water. You would not want that condition on an engine sucking sea water into the engine on shut down Safe cruising Sent from my i-Pad Chief Engineer Bob Kovach (KJ4UGE) Onboard M/Y ALLEZ! (MT50) Lying Grande Dunes Marina Myrtle Beach SC Gold Looper 5-2-2016 MTOA2631, USCG-CWOA, WRCC, AGLCA