Gas cap vent hose spewing fuel

On my last ride every time I would stop I kept smelling gas. I finally realized that it was coming out of my vent hose. When I unscrewed the cap I could hear the pressure release. I did fill the tank almost to the top before riding, probably a little too full. The one way valve is installed correctly. I did relocate the hose to the steering stem after mounting a Panoram computer. Did I create some kind of siphon action by having the end of the vent hose lower than the fuel level in the tank? It stopped after the fuel level dropped. I thought the one way valve was there to prevent fuel from escaping. While this was happening the engine would stumble from 0 to 1/8 throttle. Was the pressure forcing extra gas into the float bowl creating a rich situation? Does anybody has an answer or even a guess as to what happened?



Dont know the answer to your question but my 2000 YZ did the same thing last year when I went to Crested Butte to ride for the week, maybe something to do w/ the altitude, I dont know.


no doubt, it is the altitude, same thing happened to me...

The gas you are smelling is probably actually dripping on the ground. The one-way valve on the vent hose is designed to keep fuel and fumes from exiting the tank through the cap. Problem is, if the tank becomes pressurized, the only route for the pressure to release is to push past the needle valve in the float bowl and out the float bowl overflow. This can also cause erratic performance of the carb.

Note that since liquids do not expand, the pressure will build up more in a tank that has less fuel and more air (you know that big hiss you hear from your car's gas tank when you run it down near empty and pull in to fill it up). Changes in altitude and temperature will create a pressure differential between the air in the gas tank and the atmosphere.

On my WR250F, I have simply removed the check valve from the vent hose. It'll spill a little gas if you lay it over, but not a big deal.


This was happening when the tank was full, after I burned off 20 miles worth it stopped. The fuel was not coming from the carb overflow, I checked.

It was coming out the gas cap hose and running down my front fender. I think it was too full and the warm temps that day heated the tank more than usual. I am going to try not topping off the tank and see what happens before I remove the check valve. What you said about less fuel = more evaporation makes sense but it didn't happen that way. Weird.


Mad Potter: Could be the altitude but what I've found, especially during a hot day, is that as the engine warms and the tank heats up, the gas expands and exits out the vent tube of the gas tank. You can stop this by not topping off the tank. Maybe the one way valve doesn't work like it's supposed to, I don't know. I do know that gas comes out the vent hose because it is expanding in the tank.

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