Fuel Syphoning

Great thoughts on the probable causes.  At this point a look at the fuel line between the tanks is necessary.   I appreciate the responses.  Looking forward to many years of Coupe flying.  

Rereading of your question answered most of my thought's., other than gravity. Hard right pushes fuel left so the pump pulls from the right side the header tank overflow tube runs into the left Tant adding more fuel on top of the Fuel in the left Hence your overly filled tank...out it come out the vent.

Thank you for your suggestions.   The header tank was not refueled and both wing tanks were at the same level (approximately 3 inches below the top of the tanks) after all of the maneuvering in attempts to diagnose.   

Was the header tank filled after the mains? There is an overflow in the header tank, so you fill the header first....

Just a thought, maybe the self leveling fuel tube between the tanks is clogged/kinked and the fuel pump overflow in the header tank is doing its job and returning the fuel to one of the tanks, but then it's not self leveling between the wing tanks.

I have an Ercoupe 415C that during the pre-buy exhibited fuel siphoning out of the left wing tank’s vent hole upon movement of the aircraft. A mechanic that was on the field removed two gallons of fuel, thinking that it was an overfill situation.  The problem continued with ramp turns to the right having heavy flows and turns to the left no flow.  Takeoff run demonstrated extreme amounts of fuel being discharge from the vent, flowing across the wing.  The departure was aborted.   Two mechanics examined the plane and are baffled.  Switching the caps did not make a difference.  Has anyone experienced such a problem and what could be a possible solution?  Thanks, Mike