Gas Tank Vent Check Valve

[Birken Vogt]

Well my XR has always had a check valve in the tank vent. It appears it was made for the purpose. Recently there was a malfunction in the short hose between the valve and the cap so I took it out of the line. Then I took it on a ride. The bike has occasionally had a problem with what I thought was float sticking usually a few minutes into the ride. It occurred to me maybe it was the check valve causing the tank to pressurize when the engine heats up since it didn't do it during my one ride without the check valve. What could the purpose of this check valve be?

Birken

 

[MrLuckey]

My guess would be that it lets air in so the fuel can keep flowing out of the petcock but if you crash the gas won't flow out of the vent tube. Without a way for air to get into the tank it'll soon create a vacuum and fuel will quit flowing.

 

[Highbeam]

Yep, you can buy the valve in the aftermarket. When you dump a stock XR upside down gas will flow out of the vent tube unless you have a check valve in there.

If that valve is stuck or gummed up then your tank will be under a vacuum and won't let fuel out.

Stinks when you crash and end up rubber side up.

 

[Birken Vogt]

OK at least now I know why it is there but can anybody answer whether it could be causing my carb flooding. I have not had the opportunity to verify this yet but I suspect it might. Has anybody else had that problem and traced it to a check valve? I guess I need to make some test rides to find out. Fill the tank up with cold gas go slow and work it hard and see if I can duplicate the problem and make it go away by pulling the vent hose.

Birken

 

[ob1quixote]

I have seen gummed up check valves cause amazing problems. When the valve sticks shut, or is gummed to where the vapor exiting causes the valve to shut, and both air and engine temps are high, the pressurized tank blows fuel past the relatively weak float bowl seal. It can also allow the gas to boil, futher increasing the pressure. Of course the motor runs extremely rich as well!

The check valve in my '98 XR400 is within the cap, and I assume it is large. That allows vapor to travel 2 ways, but fuel is trapped, regardless of condition. The problems I have seen with gummed check valves have always been with the little in-line types. The gummed inline check valve doesnt have the mass to allow vapor to escape freely, and treats the rapidly expanding vapors as a liquid.

See if you can find an OE Honda cap that has the check valve in it, that will fit your tank. I havent seen one of them fail yet.

Robert

 

[Birken Vogt]

OK so does this problem that this aftermarket device intends to solve really exist in the first place. Obviously I can see why it would shut from vapors because it is mounted horizontally in the hose. But I have not had a lot of problems with my CR dumping fuel when crashed upside down and it has no such device. I always turn the bike right side up immediately anyway for several reasons: to avoid embarrasment, to stop the engine flooding, to keep oil off the piston rings, etc. But the XR is so heavy I try to keep crashes to a minumum anyway. I don't even like picking it up off the ground

Birken

 

[ob1quixote]

Your CR may have a check valve integral in the gas cap. My XR400 does.

Your best bet is to clean or replace the check valve. Clean it with a carb cleaner, after removing the rubber hoses, I would suggest Berryman's B-12. Either way, be sure to note which direction the valve goes. If you are unsure, or think it was incorrectly installed, blow strongly through the valve from both directions, the side that blocks the flow goes toward the gas tank.

If neither side blocks the flow, it is either not cleaned well, or is not a check valve {it is a swivel to keep the hose from twisting}

If both sides block the flow, it is still plugged, clean more.

The fuel tank must be vapor vented in both directions. A check valve is optional, but helps when you are upside down with the bike on top of you { a situation I vaguely remember!} An unvented tank can cause fuel starvation and flooded carbs, depending on the conditions.

Robert