Fuel Tank Breathers!!

[PeeBee]

Apologies ahead for thread-hijack nuffy spanner in the works kinda question out here regarding tank breathers...
But when you undo the cap and the hiss suction nearly sucks your hand in, is this a factory kinda thing or is there a breather issue at hand here ?

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Breather issue. The breather is a 10c item with a crude flap, designed to let air into the tank to replace the fuel that has been used. The tank does get a return fuel flow also. The tank also has a larger vent line that feeds from the filler stem to the tank to facilitate the discharge of the air during filling. If the tank hisses, the breather is blocked. I have seen first hand a vehicle that refused to start and run more than 5 seconds when the breather became filled with mud and ice. Upon opening the cap, it started first time and ran sweetly. There was a significant hiss when opening the tank.