Gas Cap vents

[MakoMike]

Originally Posted by Roger

I can't see how another vent would make the problem worse, as long as both are above the level of the fuel in the tank.

But both vents are not above the level of the fuel, that's his problem. The one vent he has now is below the fuel level in the tank, so gas "weeps" out of the vent line. Adding another vent above the fuel level is just going to force more gas out of the vent that is below the fuel level. Physics 101.

[Roger]

Originally Posted by MakoMike

But both vents are not above the level of the fuel, that's his problem. The one vent he has now is below the fuel level in the tank, so gas "weeps" out of the vent line. Adding another vent above the fuel level is just going to force more gas out of the vent that is below the fuel level. Physics 101.

I think you need to revisit your "Physics 101" because you couldn't be more wrong. By your explanation, gas should shoot out every time he opens the fill cap since he is in effect adding a vent by doing that.

Fuel weeps out because there is an air pocket that warms, thus expanding. Since his one vent is below the fuel level, the air pressure builds up and forces fuel out. If that air pocket was vented the pressure would equal the outside air pressure and there would be nothing to force the fuel out.

[MakoMike]

Originally Posted by Roger

I think you need to revisit your "Physics 101" because you couldn't be more wrong. By your explanation, gas should shoot out every time he opens the fill cap since he is in effect adding a vent by doing that.

Fuel weeps out because there is an air pocket that warms, thus expanding. Since his one vent is below the fuel level, the air pressure builds up and forces fuel out. If that air pocket was vented the pressure would equal the outside air pressure and there would be nothing to force the fuel out.

From the way he described it that is exactly what would happen is he removed the fuel cap when the boat was jacked up as he described. Your description of why the fuel weeps is absolutely correct, but what you didn't say is that when the air pocket is cool the VACUUM behind the fuel is what is holding the fuel in the tank. If he breaks that vacuum and allows atmospheric pressure into the tank the fuel is going to come shooting out until the fuel level is below the vent.

Try this little experiment. Take a can of liquid (beer soda juice or water) and poke a little hole in the side. The liquid will run out but only slowly. Now do the same thing but open the top, the liquid will shoot out of the hole. Same thing applies to his fuel tank, no atmospheric pressure on the top and it only leaks slowly. Add atmospheric pressure and the fuel is going to run out much much quicker.

[Roger]

Originally Posted by MakoMike

From the way he described it that is exactly what would happen is he removed the fuel cap when the boat was jacked up as he described. Your description of why the fuel weeps is absolutely correct, but what you didn't say is that when the air pocket is cool the VACUUM behind the fuel is what is holding the fuel in the tank. If he breaks that vacuum and allows atmospheric pressure into the tank the fuel is going to come shooting out until the fuel level is below the vent.

Try this little experiment. Take a can of liquid (beer soda juice or water) and poke a little hole in the side. The liquid will run out but only slowly. Now do the same thing but open the top, the liquid will shoot out of the hole. Same thing applies to his fuel tank, no atmospheric pressure on the top and it only leaks slowly. Add atmospheric pressure and the fuel is going to run out much much quicker.

Vacuum holds the fuel in??? If that was the case the motor would not run, It'd be fuel starved, like when the vent is clogged. Try this experiment - seal your vents and take your boat for a ride.

You're beer can example is also flawed because the hole in the side is below the liquid level, so of course it runs out due to gravity. The "atmosphere" end of the vent hoses are above the fuel level, so fuel stays in the tank thanks to gravity.

If you still don't get it I have to give up.

[macojoe]

Wow!!

Just to be clear, when cap is sealed the only way out is thur the vent, vent is blocked by fuel when boat is tilted up, allowing the expanded fuel to push up the vent hose and out.

Now Maybe I am wrong? But adding a vent in front so the vent is now above the fuel will allow the air to escape and not push gas up and out the front vent right??

Well I just ordered a new vented gas cap and will find out soon.

Thanks guys for your in put!! and we all have a opinion on how it should work, right or wrong.
But we will see soon and for those who are right congrats, for those that are wrong, consider it as a learning lesson and be glad you learned something new today! I like to learn new things all the time!