Tank Venting 101

Which way should the tank breather valve work

  • Let air IN only

    Votes: 2 20.0%

  • Let air OUT only

    Votes: 1 10.0%

  • Let air IN/OUT until a roll over then check ball or bladder closes to stop fuel from coming out

    Votes: 7 70.0%
  • Total voters
    10
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I think that it allows air in ,an as it does ,it allows pressure to escape ,when it plugs up with dirt an grime ,the built
up pressure effects the engine running ,but ,if it leaks ,then you will have fuel leakage all the time!
 
The "technical" answer is to let air "in" so as to NOT create a vacuum condition inside the fuel tank which will cause lack of fuel delivery starvation to the carb. The reason in many circumstances you will find some type of "one way" check valve system within the fuel cap is so as to still allow air in, but not allow fuel to exit through the vent. My Rovan uses a multi-staged rubber check valve system within the fuel cap gasket. Many guys just drill and tap a hose barb into the fuel cap and route a long fuel hose vertical to allow venting , but also no fuel leakage out the vent.
 
AnubisBC said:
That was my estimation as well. You would want a pressurized tank to ensure fuel delivery. A cracked fuel cap can cause a stall, because the fuel is not pressurized enough to get to the carb in a consistent flow.
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No, you do not want "pressure" built up in your tank either. Walbro type carbs use a pulse diaphragm type fuel pump that allows fuel to be drawn out the fuel tank and fed through the carb. There is also a fuel "return" line that allows excess fuel to be returned to the tank. Think of the fuel system as a constant "loop" of fuel being circulated. Some of the pumped fuel flow is routed through the carb jets and burned, the rest is sent back. Nitro engine RC's use an exhaust pulse signal to pressurize the fuel tank to allow fuel flow to the carb because there is no fuel pump.
 
@Polar, I'm not talking about a huge amount of pressure here. You still want a small amount of positive pressure in your tank. The objective of the vent hole in the cap is to allow air in to replace the space in the tank that is created from the removal of the fuel. Otherwise, you will end up with a vacuum effect sucking fuel from the carb back into the tank, and starving your engine. The fuel return from the carb also helps to add additional pressure to the tank with unspent, heated, fuel compressing the incoming air to a small degree (2-4 psi).

Pressurization is far more important to engines that do not have a diaphragm in their carb to help with the equalization of fuel delivery, but it still plays a role in the the fuel delivery of our gassers. I would be willing to bet that running your car without the gas cap will result in some issues, though not as severe as no airflow into the fuel tank.
 
Last edited:
AnubisBC said:
I would be willing to bet that running your car without the gas cap will result in some issues, though not as severe as no airflow into the fuel tank.
Click to expand...

In boats we have no return line back to fuel tanks because the carbs do not have them or are blocked off. On the original hard tanks we used to use we had vents go to atmospheric. We run 18,000 + the whole time we run around a course. Nowadays we use IV bags that are not vented to air. Carb will suck them flat from the pump. And there was no change in how they ran. Sometimes depending on engine and layout. Not uncommon to be 20,000 plus the whole time. Walbro carbs on our gas engines.
 
A fair argument against my assertions of running without a gas cap. However, IV bags don't create a vacuum as they empty. The bag flexes with the removal of the fuel, creating a positive pressure effect. The atmosphere is providing your positive pressure, much like the vented air in a rigid fuel tank would. Try putting an IV bag in a sealed tank and trying to empty it. You would create a vacuum by the displaced mass of the fuel leaving the bag with no air coming in to replace it, and would starve your engine.

Positive pressure is anything above a vacuum. I'm not talking about pumping the tank with 20psi or anything. But I will take back my previous comment about a cracked cap causing a stall unless I can prove it.
 
I'll give my 2 cents as well. The tank should vent both ways. You want neither pressure or vaccum in the tank. A little either way wont hurt as the carb pumps the fuel into it. Go look at any other fuel tank pre emissions. They vented to atmosphere with some sort of contraption to stop fuel leakage in the event of a roll over. It's really just that simple.
 
Ok, I'm glad I started this thread because I am seeing some great answers, but nothing definite. Evidently this same argument goes into manufacture thoughts also. My 5ive-B breather only allows air into the tank, yet the snappy breather allows air in and out with a check valve to prevent fuel spillage in the event of a rollover. I also believe as Sean put it, there should only be some type of tank venting. I a0slo believe the carb pumps the fuel. Maybe I'm wrong but that is the only answer for me that makes sense. A nitro engine has no fuel pump, it works of venturi effect to draw fuel into the carb so it makes sense in that aspect to pressurize a nitro tank.
 
crucialRC said:
Ok, I'm glad I started this thread because I am seeing some great answers, but nothing definite. Evidently this same argument goes into manufacture thoughts also. My 5ive-B breather only allows air into the tank, yet the snappy breather allows air in and out with a check valve to prevent fuel spillage in the event of a rollover. I also believe as Sean put it, there should only be some type of tank venting. I a0slo believe the carb pumps the fuel. Maybe I'm wrong but that is the only answer for me that makes sense. A nitro engine has no fuel pump, it works of venturi effect to draw fuel into the carb so it makes sense in that aspect to pressurize a nitro tank.
Click to expand...

When air comes in ,it relieves pressure at the same time ,the pressure is built up of gasses from the fuel ,much
like at an oil & gas refinery or your local land fill dump ,they have to install ventilation pipes to burn off the excess
gasses that build up pressure!
 
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