If you are talking about the typical low type used for mowers and hold
about 2 gallons, then the answer is leave them vented. Close the vent
if you transport the things in your car but be sure to leave some
space above he liquid or you are likely to have gasoline squeeze out
the fill hole.
In the shed, leave the vent hole open. I leave the vent open all
winter and don't notice any particular loss of gasoline over four
months (maybe I just forget, but there is still plenty of gas in the
spring and in the fall the container is usually around 1/4 full). I
leave the vent hole open all summer and don't notice any particular
loss even when the temperature is at 90 F and seldom smell gasoline
(and then, usually from a spill around the mower).
I loosen the vent cap and leave it that way to allow gas pressure to
escape and to help pouring. If the there is a lot of evaporation,
then the gas is likely stale and should be disposed--gasoline begins
to deteriorate in two weeks unless a stabilizer additive is added. I
tightly close the vent cap when transporting the gas (from the gas
station to home) to prevent spillage.
Everything seems to point to 3 months, not 2 weeks. Nonetheless, four
months seems to have no effect on a lawnmower, or a pickup. Although
I know from personal experience that old gasoline, maybe 2-3 years can
result in engine damage.
The older plastic containers have a screw-on cap over the vent hole.
Loosen the cap about 1/2 turn and the container will equalize pressure
without evaporating the gas. I noticed the new containers are
ventless so I don't know a solution for them.
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