I have an early 80's 4G63 motor that is having problems drawing fuel up to the carby.

I have replaced the fuel filter, all the rubber fuel lines and flushed the chasis lines with compressed air to ensure there are no leaks or blockages. I placed a vaccum pump onto the fuel line before it connects to the fuel pump and I can easily draw fuel from the tank

I have replaced the mechanical fuel pump and the fuel tank from another vehicle (the same as the one I am having problems with) that was starting without problems and was driven daily (until the head cracked). I even tried a new fuel pump.

I also removed the rocker cover to check that the push rod that drives the fuel pump is making contact with the lever and is actually working the pump when the cam turns.

I have checked that the vented fuel cap is ok (even tried the one from the other vehicle).

I can start it by puoring some fuel into the carby but no matter how long I keep it running for using this method, it will not draw fuel up from the tank.

Any ideas - as this is doing my head in?

Have you checked the relay in the dash? I had a similar problem but it was on a Honda Concerto. It took us a while to get to it but we got there. From then on, everything was okay.

Not sure what relay you might be referring to, as this is a fully mechanical fuel pump and system. There is a solenoid on the carby, which is working. Am I missing something?

I am wondering if in such a system the carby actually assists in drawing the fuel but I cannot see how. I thought that if a gasket or seal is leaking on the carby that might somehow contribute. However, the fuel doesn't appear to be reaching the carby yet.