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Unless you've repurposed the lines the 1/4" nipple is a fuel return to tank, not a vent. Standard configuration is 3/8" (10 mm) fuel supply, 5/16" (8 mm) vent to vapor canister and 1/4" (6 mm) fuel return to tank. Although fuel pressure ideally should be nominal, the line routes liquid fuel, not vapor.
Thanks for the pic. I definitely was thinking of the configuration on the right.