[Aussie Mike]

Thet vent point needs to be up as high as you can get it. I reckon that's your problem I'd move it to high on the filler neck if possible.

The tank vent should be plumbed up to the filler neck as well so it can purge the air out of the tank.

If you haven't got a barb on the neck bring round and I'll weld one on for you.

Cheers