New to me 22

The anti-siphon valves are more important in tanks which are inside an enclosed space, where fuel might siphon out of the tank if the line is disrupted.

In the Tom Cat, I had a bad anti siphon valve from the get go...So they can be bad at time of manufacture...

There are also check valves in both ends of the squeeze valve.

FEDERAL LAW

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 Sec. 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 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.
Click to expand...

The reality is that we don't need them in most of the C Dorys. but Federal Law requires them. In a pinch I have replaced an anti siphon valve with a straight hose barb fitting. (I always carry a couple of hose barbs fitting sized for the boat's fuel ports, including the Racor Filter. Just in case.

I also had one case where I had used teflon tape on a fuel connector and a small tab of the tape got into the fuel flow area--was causing an intermittent obstruction to fuel flow. There can be cracks in the pickup tube which cause obstruction. Also there is a filter on the bottom of the pickup tube which can be fouled with debris.
 
I have a 2010 and was experiencing running out of fuel when clearly there was
plenty of fuel in the tank. Primer bulb would occasionally prime. My suggestion
would be to replace the fuel pick ups AND anti-siphon valves. The fuel pick ups on my boat had disintegrated to the point that one was 2" long and the other 6" long, so would not reach the fuel. This is very common after 10 years. I ordered replacement parts for the Moeller tanks online only to find that our dealer and West marine both kept the parts on the shelf. Let me know if you need help with part numbers have questions regarding removing the panels
in front of the tanks. While you are in there looking at the tanks, be sure the vent hoses have not collapsed as one of mine had.
 
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