Sounds like you are on the right track, John.  Yes, “T”ing the high pressure return with your low pressure return could potentially end up sending some fuel toward your Webber.   Idle rpm  is when the fuel injector pressure regulator is dumping the most fuel back into the return line – and it dumping a lot.  At idle, there is minimum fuel being injected into the engine, so therefore, the EFI pressure regulator has to release the maximum amount of fuel back into the return line to keep the pressure regulated to what ever value you have set it.  At WOT there is much less fuel being returned as more of it is being consumed by the engine and there less fuel is need to be released by the Pressure regulator to maintain pressure. 

 

Now I see you have a pressure regulator in you Webber line, don’t know what it is set for – but as I recall it only takes a few PSI of fuel pressure to feed a carburetor float bowl – so with the EFI pressure regulator dumping a whole lot of fuel into the return line – it may well raise the pressure to a couple of psi in that line which pushes back to the carb float bowl thereby providing an unintended path for fuel to the Carb when the injectors are operating.  Could possibly be a faulty low pressure fuel regulator that is letting some fuel seep – I would think that if it sees no fuel pressure from your low pressure pump it should have its port to the return line closed off which theoretically should keep fuel from backing up through the return line to the Carb. But, one of two things could be happening there – you simply could have a leak in the low pressure regulator or if there is no fuel pressure from the low pressure pump – perhaps the return port on the low pressure fuel regulator remains Open.

 

In any case, looks like you have nailed the source of the problem down.

 

Ed