ThermoQ -2 questions:

EL5DEMON, that is a good point and would probably be valid on an older carb, but my carb is new -well, better than new actually (as a 35+ yr old 'new' carb would still be prone to seal breakdown). Mine has been properly rebuilt by a guy who knows TQ's inside and out. So he told me he has re -bonded that well area.

JimmyRay, thanks for those very clear diagrams -it has got me thinking actually -you may be right about the vacuum created etc. Maybe my float bowls are not vented and so it will suck fuel instead of air when the height levels fall. There is a very small brass tube sticking out of the side at an angle halfway up on the RHS of the carb. I have blocked that off - i wonder if that is the bowl vent. I will have to look at some diagrams. But that might explain it. BTW, my 340 with AVS in a previous car was terrible for this problem as well. Also, another fellow told me that Carters were well known for draining their fuel away over a few days