Gas Tank Vent Check Valve

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.

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.

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

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