If it's a uniflow (three lines from the tank) then you should cap the overflow tube, but that's as much to make the uniflow work correctly as anything else.

Jack,
I sounds like the airflow over the open vents is creating a suction an pulling the fuel out.  In the "bad" old days, we just cut off a piece of fuel tubing at a 45 degree angle and slipped it over the vents with the angle facing forward.  Today, I plug the lower vent and route the upper vent to a piece of copper tubing bent 90 degrees to face into the airstream and located on the inboard side of the fuselage just above the top of the tank.
Steve

Thanks all. Will go and try again