How to check fuel tank vent/ anti syphon valve

[candiaoh]

There is a lot of talk about fuel delivery issues. Two areas discussed are checking the fuel tank vent and anti syphon valve. Not to sound stupid, but what do these look like and where are they located (besides the obvious answer- on the fuel tank). I have a manual that goes into great detail about what happens to fuel as it enters and leaves the fuel pump, but really not much before that. Specifically, where on the tank is the vent and where would I look for an obstruction? Is it on the tank or do I follow fuel lines back up to another location. If someone could clarify this, I would greatly appreciate it.

Thanks!

 

[pantaloonz]

Hi Candiaoh,

It's going to depend on your boat. I'm in the middle of doing work on mine right now, I'll attach a pic later on (unless someone beats me to it). If you have an I/O which I'm assuming you do you gotta pick the floor boards up over the tank to see it. On my rig, two hoses are attached the tank, one for filling (larger) and a smaller hose is the vent.

prlly will look similar this:



So to answer, then vent attaches to the top of the tank, and and obstruction could be anywhere in the tube.

Good thread on iboats (linked below)

I'll be disconnecting mine this evening and running a stiff wire through it (it's really just a straightened out coat hanger, but don't tell Bondo) I'll check the connection for dirt and grime build up and give that a once over as well.

-Pantz

 

[candiaoh]

Thanks- I do have an I/O. That does help a lot...that being said, where is the anti-syphon valve located? is it inline in the main fuel inlet?

 

[pantaloonz]

Greets,

The antisiphon should be in the device located just off the fuel pickup connector (top right). In the included picture the valve should be in the fitting right between the end of the tube and the port holding the fuel pickup. On mine (I assume most are similar) was a small ball bearing looking thing which when depressed should move easily along the line of the fuel delivery. My valve had been replaced not long ago it was nice and shiny and moved with ease.

Please be very careful around your tank connectors and for good measure disconnect any batteries and eliminate ANYTHING that could cause a spark.

Also be careful if you decide to check the fuel pickup line, if you mess up that connection it's not going to a fun fix.

-Pantz

 

[candiaoh]

Will do. Very Helpfu!

 

[candiaoh]

One more thing...just trying to understand these fuel systems....I get that if a gas tank is not fully ventilated in will cause gas starvation. How can a faultly anti-syphon valve do the same?

 

[pantaloonz]

Hi


The vent does 2 things let's fumes go helps regulate the fuel flow preventing starvation as you identified

The anti-siphon valve stops fuel from sucking back towards tank. (siphoning from the engine/lines) But if it's jammed up/clogged/stuck fuel won't go the other way either! Damn Valve!

Here's a nice snip that sums up nicely (this link may help others too)

""If the fuel tank cannot breath, it will cause a reverse suction condition and prevent fuel from getting to the motor. The inlet hose and fittings should be checked periodically""

http://www.******************/iofuel.asp

 

[pantaloonz]

oops post didn't like my link... I sense a moderator may tell me I made a boo boo.

 

[jbart90]

The anti-siphon valve is very similar to a simple check valve. Its purpose is to prevent fuel from starting a siphon out of the tank and into the carb/engine compartment. This could cause a fire or continue to feed a fire by dumping fuel when the engine is off. It is especially a concern on larger boats where the fuel tank may be mounted higher than the fuel inlet to the carburetor, which would allow a siphon to start easily.

The fitting itself has a spring and ball bearing. When the engine is running, the fuel pump suction overcomes the spring force, and pulls fuel past the ball bearing. When the engine is off, the spring and ball bearing prevent fuel from siphoning out of the tank, and stops fuel in the line from emptying back into the tank (helps to keep lines full of fuel)

If this fitting gets corroded, the ball bearing may stick, and prevent adequate fuel flow.

 

[southkogs]

... I sense a moderator may tell me I made a boo boo.

Grab ya' a lottery ticket Pantz ... yer' Jedi sense is workin' :bounce: