Are tank breathers really needed?

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Feb 26, 2017
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#21
Because in the heat the can self vent, but when they cool they do not allow air back in.

You won’t see this happen with a can of trufuel.
The can is completely sealed, put it in the sun and it will blow up like a balloon, put it in the air conditioned shop and it shrivels up, I know this because it's a bit of a pain in the neck.

I'm going to try shaking next time I have the opportunity. I don't think it will work, but I'm not certain enough to just say it won't work without trying it.
 
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#22
Run your ski with a working vent and then try it without one. I just had my blaster vent go bad (letting air in and out) and while she ran okay it felt like the jetting was off. Replaced the valve and was a noticeable improvement. Then again maybe I could have re-tuned the carbs to run great without the valve?
 
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#23
The tank should be under pressure due to expansion of fuel vapor. Maybe not much, but there should be some pressure there. conzider taking a sealed can of fuel, and shake it around. The fuel can will start to gain vapor pressure from fuel evaporation due to being agitated. Also the check valve is there to stop fuel vapor from escaping otherwise it could build up in hull and explode lol. Also from a performance stand-point this fuel vapor would eat up available oxygen - and eventually choke and stall the engine out if enough vapor gets into the hull. If you've ever forgot your gas cap you would understand. Venting fuel vapor is an explosive hazard. The check valves should hold up to about 4psi or so.

Secondly I'm a firm believer that the expansion pressure in the tank helps with fuel scavenging - which is very important for our mechanical fuel pumps. Pressure built up in the 'top' of the tank helps the pump syphon fuel from the tank, as this pressure is helping to push the fuel up into the line and into the low side of the fuel pump<s>.
The gasoline will evaporate or condense based on the temperature, volume, pressure, but once it's at steady state, it all stops. Now, add to that that you start drawing a vacuum by sucking fuel out. The fuel has to evaporate fast enough to match that rate for there to remain some positive pressure. Since gasoline so readily evaporates at summer temperatures, and apparently will continue to do so until at a pressure above ambient air pressure (hence gas cans expanding, etc.), then maybe it can keep up with the vacuum introduced by the sucking out of fuel. But on a cool day? Not so much. I guess the best approach is to put a gauge on it and know for sure.

Regarding all the bits about keeping fuel (liquid or vapor) in the tank: I keep forgetting that the OP really asked "are they needed at all." I just honed in on the pressurization bit.
 

SUPERJET-113

GASKETS FOR CHAMP BRAP!
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#24
Ive had a check valve fail once on a SJ and it sucked my gas tank in to half its size. I dont even use those valves anymore and if I did I would run 2 in a "Y or T" configuration. I just loop hose in about a 4" circle 2-3 times and zip tie off. So I will say no they aren't needed if you dont want them.
My .02 lol
 
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Vumad

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#25
So interesting fact.

When I go riding at the lake, I fill mason jars with ice water to refill my bottle so as to save plastic. It was impossibly hard to remove the caps at the lake. It took me a while but I figured out why. When water freezes, it expands into ice. Consequently, when it melts, it shrinks to water. My jars became impossible to open because they actually pulled a vacuum as the ice melted. Makes sense, since that is how they sealed the pasta jar in the first place.

Anyway, containers behave differently under pressure and vacuums. Just because your jug is sealed, does not mean it's not leaking. Its performance under a vacuum is probably much better than its performance under pressure.

A theory could of course be tested...
Seal a can in the AC, put it into the sun and then back into the AC.
Seal a can in ambient and then move it to the AC.
Seal a can in ambient then move it to the sun and then back to the AC.
You'd still have to have a way to test for leakage at pressure, wether using a gas meter, a bag or soapy water.
My theory is your sealed can is leaking under pressure but not under a vacuum.

Anyway, just rambling at this point because that's what I do I guess.
 

yamanube

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#27
If you allow your fuel tank to vent straight into your hull, you're going to have a bad day. You can't completely seal it or you won't be able to maintain fuel pressure/flow.
 
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#28
pressure in the tank is not a necessity for the carbs to pump fuel.as many have stated,there are plenty of vehicles that use diaphragm carbs with vented gas tanks.
also,keep in mind that even though the tank is under pressure and fuel gets pushed to the carb,when system is closed,the return fuel line encounters this pressure also,negating the pressurized fuel flow .
 

bored&stroked

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#29
Fuel pumps work much better pushing then pulling. Pressure in the tank helps the pump get the fuel from the tank to the pump itself where it can more efficiantly push the fuel vs pulling.
 
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#33
I always set up my 2 stroke skis like old Seadoos. Use (1) inlet check valve vented to dry intake area. And (1) 2 psi pressure relief check valve vented overboard. Any excess pressure from thermal expansion or turbulence in tank is vented. So the tank holds right at 2 psi consistently. Jetting is always consistent since fuel pressure isn’t changing with tank pressure. So set your fuel pressure restrictor once then your good. Works great

Interesting enough on the new Yamaha FXSVHOs the tank intake vent actually goes through a water separator as well which is nice. However there is no relief vent. So when you fill it up, sometimes gas blows back out the filler right towards you. They need a simple relief valve on tank to solve that.

Also don’t forget USCG requires closed loop fuel systems.. So no open vented float bowls or gas tanks that can leak causing explosion hazard in hull.
 

long beach local

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#36
Ive had a check valve fail once on a SJ and it sucked my gas tank in to half its size. I dont even use those valves anymore and if I did I would run 2 in a "Y or T" configuration. I just loop hose in about a 4" circle 2-3 times and zip tie off. So I will say no they aren't needed if you dont want them.
My .02 lol
I had 2 NEW Cold Fusion fuel tanks split open when I took my ski to where its really (Hot) and they were replaced by Cold Fusion free of charge. I have now been running both of my skis for several years now with No check valve at all, they run just fine. I have a long hose coiled up and tucked away on my surf ski and water intrusion is not an issue. The other ski runs a long hose up then down the handlepole and vents outside engine compartment. My .02
 
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