It is the way they are designed and what filler nozzle at the pump you are using. Some on here have used filler tubes off a diesel truck and say they work better. My plan on my 74 when I get to that stage is make my own, I will more than likely have to experiment with a couple of different ones, but I have had if with the trickle fill up. I have had mine apart several times, no clogs or vent problems, just wont take gas very fast.
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