Educate me on my gas tank please

Joined
Sep 5, 2007
Messages
1,790
I want to know the purpose of the vent screw(located on top of cap) for my aftermarket gas tank I use with my outboard. Its a 1960 merc 300 if that matters. thanks
 

emckelvy

Commander
Joined
Jan 16, 2004
Messages
2,506
Re: Educate me on my gas tank please

That vent screw is supposed to be loosened by turning in a CCW direction when you're running the motor, and tightens down to seal vapors in the tank when you're not using it.

If you were to leave the vent shut while running the motor, eventually the vacuum caused by the motor pumping fuel out of the tank without an air vent would overpower the fuel pump and the motor would starve for fuel.

One thing about those aftermarket tanks, if it's plastic and you're gonna leave it in a very hot place (which I wouldn't recommend, BTW), I wouldn't close the vent off completely. Pressure under a hot sun could build up and cause the tank to bulge, compromising its structural integrity. Better to move it into the shade somewhere. But normally you'd close the vent so the fuel is not subject to sucking in water vapor (which your motor will really not like!).

HTH............ed
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Educate me on my gas tank please

When the engine's running it's drawing fuel from the tank. That fuel has to be replaced by something. That something is air, and it comes in via the vent.

Chris.........
Edit: Hey guys, the OP is 'thefairlaneman', he's been around for years... I think he's having a lend of us. :D:D:D
 
Joined
Sep 5, 2007
Messages
1,790
Re: Educate me on my gas tank please

Thanks you for the information...The engine kept cutting off and I didnt think about the venting issue untill later.Now I will run it and be sure the vent is open.
 
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