That's an interesting problem. Where are your fill and vent fittings on the tank in relationship to each other? If the tank is not level and the fill fitting is uphill a bit, I can see how that might cause a problem.
I ran a boat for a while that was always burping fuel. The fill fitting and the vent fitting were at one end of the tank. When the boat was in the water, the other end of the tank was high (towards the bow) As the tank filled, the aft end of the tank filled up and closed off an air pocket in the forward end. The air bubble created pressure. The only way to put anymore fuel in that tank was to move weight into the bow.
1983 Fraser 41
La Conner, WA
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Boating for over 25 years, some of them successfully.