gas tank vent hose check valve-what's the skinny?

a spin off of my post about crappy graphics (read it if you don't know what i'm talking about)

what's the advantages / disadvantages of running the little check valve in the gas tank vent hose? i run the yamaha mostly on the track now, and am considering installing one on my xr200.

any input wanted.


racer36 :)

It's a one way valve. Lets air in but does not let fuel out. In a crash it doesn't let gas leak out the hose. Running it has never cause me a problem but it sounds like, from the other topic, it could.


[ September 05, 2002, 05:57 AM: Message edited by: jbird ]

Advantages, no fuel will dump onto the ground.

>> I FIRMLY believe this is all EPA crap, just like smog crap on your car, oxygenated fuel (which is not good), vapor extractor canisters on the gas pumps (ever see the warning labels DO NOT TOP OFF FUEL TANK?), MTBE additives to gasoline.


Pressurizing your gas tank. Your carb is gravity fed from the gas tank. As the float drops upon fuel demand, more fuel is gravity fed from your tank.

Now, pressurize your tank. The pressure can overide your float or your float drops on fuel demand >> FORCING fuel into your carb. Now your carb is PRESSURIZING with fuel. When your throttle is opened, your carb has become a fuel injector (or acts like your accelerator pump per-se). Your intake valves open, and fuel is FORCED, not vacuum dragged, into your engine. Your bike floods out and stalls, or fouls a plug. This is real crummy while trying to clear a double, or hammering through whoops, or flying down asingle track, or driving in traffic to your favorite riding area. Just ask Dougie Henry about stalling on a mx track, when he broke both his arms.

Oxygenated fuel can have this effect >>

Riding at higher elevations, when the gasses come out of solution in your fuel (Henry's constant).

Oversize gas tanks w/ the bottom of the tank near your exhaust pipe.

Aftermarket pipes which come closer to your stock fuel tank.

Increasing ambient temperatures causes your fuel to expand.

>> (On my submarine in my Navy days, our Turbine Generator Lube Oil System (TGLO), would go from 155 gallons to 195 gallons when going from 70F to operating temperature of 128F. Obviously this isn't the same, but this is a fine example of expansion due to temp increases).

There have quite a few TT'ers (WR owners) that have been flabbergasted w/ stalling problems, fouling problems and poor running problems. After all the troubleshooting, this stupid check valve become the only thing it could've been.

Guess what?

:)IT WAS!!!!!!! :D

Now, you can decide on your own what you would like to do. I know I spend a lot more time w/ my bike upright, than on the ground. I do plan on keeping it that way too ! :D

[ September 05, 2002, 06:47 AM: Message edited by: NH Kevin ]

Dear Kevin.....

After all your "Soap Boxing" this issue I decided to do a little experiment last night.

I took my portable inflator and charged my tank with air ...just to see if it held pressure as you aluded. I injected 10 lbs of air into the tank and waited 30 seconds....I checked the pressure again and it fell off to less than 1lb, Next I recharged and checked the pressure after 45 seconds....Zero lbs pressure.

To verify my experiment I did the same experiment on my WR with a 2000 YZ tank....After 30 seconds the pressure was zero....

It appears my friend that you may have a bad valve on your hands. Mine will let air escape but will shut down when filled with liquid....(I tested that as well on both valves).

Bonzai :)


I myself had never had a problem, up to the point when I removed the check valve.

Numerous TT'ers have talked of removing their gas caps, surprised to find a pressurized tank. Almost always they are running ovewrsize tanks or different exhaust systems.

These Guys also have had running problems w/ their bikes.

Maybe they all had bad checks...or maybe YOURS is bad...???

I dunno...

All I know is the Gov't isn't ALWAYS right! :)

There are check valves that will allow air to escape slowly if the pressure builds up slowly. I JUST finished filling my Daughter, Krista's, waterbed. There is a check valve you can install that will bleed off any air that is slightly above ambient pressure, but will not allow water to flow out. There is a tiny bleed port that allows air to piss out. If a mad rush of [air/water/fuel(?) ], the orifice cannot pass the volume of fluid, and the check valve is seated. This also closes off (blocks) the port.

Like I said,I don't want to find out I have a problem, when riding w/ Guy Cooper and clearing the 150' sextupple jump at his house in OK! :D

I hear you...

One thing these guys need to remember though, is that maintenance is required even on the overflow tube. If you get junk in the line...(I've seen many that got grease packed in the line when servicing the head bearings, and even dirt in the lower end of the tube) that will affect the operation of the check valve.

I'm also a boater and have check valves between my dual tanks and the breather ports on the sides of my boat. They work exactly the same way as the check valve on the Yamaha. Designed to let air flow in both directions but when liquid races to the valve it is cut off up by the closing valve.

I seriously doubt that my valves are bad, as they are doing what they are supposed to do. But there are other variables here that could be causing problems with other folks. Not knowing how their bikes are maintained, or even if they accidently installed the valve upside down (I've seen that 3 times), or there could have even been a bad batch of these things during the manufacturing process (been known to happen).

I would recommend manually testing the valve if someone is concerned. It's easilly done by removing the hose and valve and blowing into the long hose side. Air should be allowed past the valve. Then blow hard through the check valve end...The valve should lock blow gently through the check valve side and air should escape.

anyway my 2 cents....

Bonzai :)

that what i thought yamakaze.

in theory, gases, unless in extremely high volume or velocity, wouuld not be able to close the check valve in the line.

that and the fact that when you sit forward on these beasts (especially while warm with the motor off) you can hear the tank venting while air escapes due to applying pressure to the tank (be it by sitting up by the gas cap or squeezing the tank with your knees.

whether it hampers vapor flow enough that the vapors need to find another venting route (ie. thin tank plastic) or not, i don't know, but this would be interesting, yet very difficult to measure on two bikes, one with a valve, one without, to see which had the graphics bubble or peel worse. there would be far too many constants to keep check on (temp, riding conditions, crashes, frequency, graphics manufacturers, etc...) to make this worth doing, only to find out that the valve inhibited 2.3 more miniscule bubbles per riding season than without... :):D

thanks for the posts guys. i think i'm gonna put that on my honda. and i'll probably leave it on the yamaha for now, too.

keep posting if you think of anything else that might be relivant to this.

racer36 :D

Here is some more vent valve info for ya.

Kinda goes against what was said above but on a few ocasions when it was damn hot outside I have opened the gas cap to let air escape. I could hear it trying to escape through the vent tube. But when I opened the cap alot of air escaped. Interesting.

I have seen a really cool vent. It is a brass vent with an o-ring swivel. It dont seem like much of a big deal. But when you open your gas cap the vent swivels so the vent line wont twist around. Its actually really nice. And it helps eliminate the chance of your live getting twisted and cutting off the vent.

I had a friends xr650 twist his line by accident. The bike would keep dying every so often. Took awhile to figure out that the tank was not venting and therefore not able to flow gas to the carb.

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