i have had a similar problem on my 1600 rally car with twin webers. Engine would run fine for indefinatly with no load (idling) but would starve of fuel when throttle is applied for more than about 30 secs.
Turned out to be a blockage in the fuel line. Try disconnecting the pump and connecting to the line to the tank and blowing down the fuel line from the engine bay with an air compressor.
You should hear a good bubbling sound in the tank if its flowing alright