Is air blowing past the filler or out of the vent/breather. If it's the filler then you need to check that the breather isn't blocked. Take out the one way valve (you should really need this if the vent that the breather is attached is high enough above the tank). If unblocked then what size is the breather tube! Should be at least 25mm Dia or bigger. If the fuel's comming out the breather vent, then I suspect one of two reasons.
First: How much higher is the vent above the top of the tank.
Second: Check that where the breather pipe enters the fuel tank that, it's at the top of the tank and that there's no piping inside the tank. The purpous of a breather is for air to escape.
Looks Slow but is Fast
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