Are tank breathers really needed?

maxboost

Super tuned
Location
Ia
From a performance stand point,

So to me a tank breathers purpose is to let air in to the fuel tank and not let air/fuel out?

The fuel tank is not pressurized, there is only fuel pressure from the fuel pump to the fuel restrictor.

Correct me if I’m wrong please

Ongoing debate with a friend that rides a 550....
 
everybody that I consider a guru says the one way valve is not necessary.
I always shake my head when the first answer to most problems is replace or check that valve.
maybe tank pressure was needed to help the very weak fuel pumps on the old mikuni round body carbs back in the dinosaur days
 
There are two different things people are talking about here.

One is whether you need to vent the tank at all, obviously you do or there will be a vacuum in the tank as the fuel level goes down. The tank can be vented with no check valve, just a vent to atmosphere. Only downside I can see to that is nothing prevents fuel from spilling, or water from getting into the tank.

The other is if you need a positive pressure in the tank, which I have seen some people claim you do. I don't see why (or how) this could be. I'd be thrilled if somebody could explain how it's even possible. Some people say the check valve is to hold a positive pressure, I'm saying I *think* it's just to keep fuel from spilling and water from getting in.
 
Location
West MI
Regarding the positive pressure concept, there's nothing to pressurize a tank with a one-way air valve (allowing air in, but not out) other than 1) atmosphere 2) the fuel pump return line. Since the quantity of fuel being returned via the fuel return line is LESS than is being drawn out of the tank (and there's no additional air in that fuel return coming from the carbs), there is nothing to cause the tank to get pressurized. This, of course, is all while running. Ambient temperature change from cold to hot will of course cause the tank with a one-way (air in only) valve of a non-running ski to get pressurized, but after running for a period of time, that pressure will be reduced back down to just ambient air pressure. Just trying to state what I think others here have said in a more specific way.
 

Zero Client

BeerrroooOOOT!!!
Location
Douglasville, GA
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>.
 
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Jcary85

Site Supporter
Location
Glenmoore pa
This is pretty interesting. Every time I take a fuel feed line off my tank (even hours after the motor last ran) it is pressurized. There is enough pressure sometimes that the fuel pours out for 1-2 seconds (I've been accidentally covered in gas several times). I wonder might cause this kind of pressurization? It is way too much that its just temperature changes.
 
I have a vintage snowmobile that runs a diaphragm carb similar to the old round body mikuni bn (in fact I have run a round body bn watercraft carb on it in the past) it pumps fuel from a tank that is vented straight to atmosphere with that old carb with supposedly a way less powerful fuel pump than the square sbn. Makes me doubt that an sbn needs any help.
 

Zero Client

BeerrroooOOOT!!!
Location
Douglasville, GA
I have a vintage snowmobile that runs a diaphragm carb similar to the old round body mikuni bn (in fact I have run a round body bn watercraft carb on it in the past) it pumps fuel from a tank that is vented straight to atmosphere with that old carb with supposedly a way less powerful fuel pump than the square sbn. Makes me doubt that an sbn needs any help.

I have the same snow mobile design, but in a Honda Odyssey. Same thing really. I put the fuel pump below the fuel line so that gravity helps feed the low side of the pump. Just curious - is your snowmobile's pump above or below the fuel line?
 
The fuel pump is built into the carb, fuel pump section of the carb is on the bottom, fuel comes in at the very bottom. The carb I'm using right now is a walbro but I have literally used a bn38 watercraft carb in the past.
 

smoofers

Rockin' the SQUARE!!!!
Site Supporter
Location
Granbury, TX
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>.

This post is spot on. All of it.
 

Vumad

Super Hero, with a cape!
Location
St. Pete, FL
I may be wrong but this page covers my understanding of how pressure changes affect volumes.

https://industry.airliquide.us/volume-compressed-gas-cylinder

16.7psi x 3ga = 14.7psi x X
50.1 = 14.7 x X
50.1 / 14.7 = X
3.4ga

3.4ga - 3ga = 0.4ga

So when the pressure of the fuel tank increases from 0 to 2psi above atmospheric, the "air" contents of the tank can potentially displace 0.4 gallons volume (air or fuel) through the fuel pickup.

I'm not sure that this translate properly when discussing pneumatics with hydraulics, since the formula is addressing filling a compressed air tank, but the concept applies.
 

smoofers

Rockin' the SQUARE!!!!
Site Supporter
Location
Granbury, TX
If fuel vapor causes pressure why do my sealed gas cans implode when I take them from hot outside weather into my air conditioned shop?
Because of the ideal gas law. As the vapor cools it reduces in volume (thus, reducing the pressure inside the can). Take your imploded cans and violently shake them, you should see them start to pop back out.
 

Vumad

Super Hero, with a cape!
Location
St. Pete, FL
If fuel vapor causes pressure why do my sealed gas cans implode when I take them from hot outside weather into my air conditioned shop?

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.
 
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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