REFLECTOR: Fuel venting issue

[Tom]

Took first long flight with plane (FG/STD) this past week. When 
refueling left tank in Huntsville fuel started pouring out of the vent 
hole at a fast clip. It only started when about an inch from the top but 
it worked like a siphon and lost 15 gallons. Filled it up to about 2 
inches from top and no issue at all, tried topping off and it started 
doing it again. I filled it in Fort Myers before departing and there was 
no issue. Any ideas as to the cause?? Not sure if this is pertinent but 
the sump tank does NOT have a vent line. thx Tom