West marine carries a check ball for the fuel vent line. You may be able to
use larger lines or change where the vents are connected to the tank as
well.

"Dave Isherwood" wrote in message
...
Good Evening's All - Much appreciate (incremental) remedies.

Comfit is a Carver 520 with 2*400 gal. tanks. There are two vents on the
sides (2 per each) that will vent diesel after filling 10 gal. into 200
gal void.

I just spent 1.5 hours putting 200 gal. into each of two 400 gal. tanks.
We had to put a cloth over the two vents to stop diesel going into the
water.

Do you have any suggestions on how I could fix this?

Dave