[fortrdb]

This has been discussed before, but I thought it important to revive this topic based on a recent experience I had on both my machines. Thanks to this forum, I was aware of the potential for this problem to manifest itself and what to look for as an indicator.... fuel tank outgassing as the fuel cap is removed, aka outward hissing as you release the gas cap.

We did a 140 mile ride that required gassing up in Sonoita. No big deal....done it before w/o issue. This time, both machines exhibited the problem at the re-fuel point. My machine with 13.5K miles was worse. My second machine with 11.5k miles was not as bad as I have thought about this situation. Fortunately, I re-fueled the guest machine first as it had less pressure built up. This alerted me to the problem before I opened up mine. I did not initially realize the problem was occurring on my second machine until I had fully opened the cap. It did not spew out on me, but the outgassing of the boiling fuel was so intense that I had to wait about a minute with the cap off before being able to add new fuel without it being tossed back out the fill tube. It took probably three minutes of gradual cap opening on my machine and then another two minutes of cap off time before being able to put new gas in without it being thrown back out...... Riding a potential bomb essentially.

The one way valve in the vent issue appears to be failing at about 11K/5yrs based on this experience. Blowing through the vent tube on my guest machine provided venting for the most part in a static environment. I could get it to lock up once in a while. Add some vibration and I am pretty sure that supports malfunction of this valve couple with time. My machine was flat out blocked when I tried blowing though the vent line. I could hear the valve click when I pressurized it, but absolutely no flow. This explained the pressurization I was seeing after an extended ride.

Based on @azyxzer recommendation and other site guidance on this topic, I have implemented a different fuel tank vent system that uses 1/4" vacuum line running to the bottom of the B pillar, up, over and down the driver side with a fuel filter on the end to prevent dust from going back up the vent line. I think this will prevent any further over pressurization issues in the future.

Fuel Tank Vent Issue

Fuel Tank Breather Hose Joint …. AKA Roll Over Valve / check Valve ! There have been several post’s about installing a fuel filter in the fuel tank vent line due to pressure building up in the fuel tank, some people said debris in the vent line was causing the RO valve to malfunction. I...

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[danielplace]

Most fuel systems don't vent. They often use a check valve. It lets air in not out. They are almost never a wide open vent with emissions requirements.

Have you studied all the components of the venting and rollover valve I think it may have to understand how it all functions.
I know it has charcoal canister I think with a rollover valve and then a solenoid that evacuates the canister vapor into the engine.

They are likely to get pressurized even when new as has been reported.

The trick may just be a cool down time after use before pulling the cap. Hot fuel is gonna boil. Without a free open vent these don't have it is going to build pressure. Just the heat never mind the fact gas boils at only 120°. Leave your sealed gas can in the sun/heat. Rectangle turns into a ball. Imagine shaking it violently the whole time too.

 

[fortrdb]

To your point, there is a one way valve designed to vent with a small positive pressure, like 1 psi. Not sure if it has a charcoal canister or not. So as you say, it does not sit open and vent to the atmosphere unless the tank it producing enough positive pressure. That valve is the malfunctioning point and I am trying to avoid the situation of a highly pressurized fuel tank which is not what the intent of their design is. This just makes it significantly more reliable. I will lose some gas to open venting. I think the filter on the end of 15' of line will help to minimize that loss. We will see how this plays out. I routed line over 3 perpendicular planes and down to below the vent output level, so no chance gas comes out in any orientation.

 

[danielplace]

Nothing wrong with your new venting. It will work wonderfully. Everything was vented that way before EPA said they can't do it no more.

I was just saying the factory setup may build pressure even when working as designed because there may be a design flaw. It can't vent the amount of pressure buildup the tank is producing. Same thing happening on new ones and same with the Wolverines with similar venting. The fuel gushing out when opened hot has been reported many times from these Yamaha vehicles of all ages.

I recently got rid of the charcoal canister and factory venting on my Scag mower and ran it to a fitting I put in the roll bar. It is all good but the gasoline smell has gotten way worse in the shop in the morning when I come in the little door you have to hold your breathe until you get the overhead door open.

 

[fortrdb]

The valve (Pipe Joint Assy) design could indeed be suspect. The stock vent system does not go back to the intake. It is a simple dump to the atmosphere. You can feel it if you just blow on the vent tube. It clicks open at fairly low pressure and then flows freely.....most of the time. I have the "luxury" of having a sample of two to check out. One was opening intermittently and the other was clicking, but not flowing AT ALL.

 

[danielplace]

I see. Mine is a 2019 and it goes to canister and to the throttle bodies.