Which jerry cans? The US variety with screw threads or the euro can with the cam latch lids. Have a few of the euro cans and had the same problem. Finally found the "correct" military spout at swissarmyvehicles.com (standard disclaimer) has a built in air vent and empties a can quickly. The down side is it is expensive, cost more than the cans and will not fit the restrictor plate in the gas tank of a car.