The only one they had at my local shop was an anti siphon valve version. Is this really necessary for an outboard application?
Here's the rule:
§183.568 Anti-siphon protection.
Each fuel line from the fuel tank to the fuel inlet connection on the carburetor must:
(a) Be above the level of the tank top; or
(b) Have an anti-siphon device or an electrically operated fuel stop valve:
(1) At the tank withdrawal fitting; or
(2) Installed so the line from the fuel tank is above the top of the tank; or
(c) Provided that the fuel tank top is below the level of the carburetor inlet, be metallic fuel lines meeting the construction requirements of §183.538 or “USCG Type A1” hose, with one or two manual shutoff valves installed as follows:
(1) Directly at the fuel tank connection arranged to be readily accessible for operation from outside of the compartment, and
(2) If the length of fuel line from the tank outlet to the engine inlet is greater than 12 feet, a manual shutoff valve shall be installed at the fuel inlet connection to the engine.
[CGD 74-209, 42 FR 5950, Jan. 31, 1977, as amended by CGD 81-092, 48 FR 55737, Dec. 15, 1983; CGD 85-098, 52 FR 19729, May 27, 1987]
Here's the disclaimer:
Title 33 NAVIGATION AND NAVIGABLE WATERS
PART 183 BOATS AND ASSOCIATED EQUIPMENT
Subpart J—Fuel Systems
Source: CGD 74-209, 42 FR 5950, Jan. 31, 1977, unless otherwise noted.
General
§183.501 Applicability.
(a) This subpart applies to all boats that have gasoline engines,
except outboard engines, for electrical generation, mechanical power, or propulsion.
(b) [Reserved]
[CGD 74-209, 42 FR 5950, Jan. 31, 1977, as amended by CGD 81-092, 48 FR 55736, Dec. 15, 1983; USCG-1999-5832, 64 FR 34716, June 29, 1999]
I disabled both of mine when I installed a new tank.
Good luck.