1985 mariner 45 starts but dies?

Hi,this engine has been stored for last 2 years my friend has bought it for his day fisher.We have replaced all the fuel pipes,plugs,cleaned the carb bowls&filters and the main engine filter and replaced the boats fuel pipe and primer bulb.Once primed it starts easy and will run great if theres plenty of throttle on,if the throttle is eased of it dies.If you re-prime it with the bulb it starts straight back up but still only runs if you keep the revs up(too many to be putting it in gear) Its a 4cyl with twin carbs and i cant see a fuel pump anywhere,its definitely dying through fuel starvation but we cant see why ,it will run on slightly less throttle if the fuel tank is raised higher than the engine,its a new tank and yes we open the vent.any ideas would be very welcome,thanks.