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are you sure that this is a 1979 car? as fuel injection on the old air cooled beetle wasn't introduced until 1986?
in that case I'd check that throttle is opening fully and if that's OK then I'd check over the state of the injector wiring harness as it sounds like that you may have 1 or more injectors with a broken wire - this can be checked with a 'noid' light
and disconnect the loom on all 4 and trace it back checking for hard or damaged sections and corroded joints
if you can't find any issues then I'd use a meter to check the resistance / continuity between the injector plug and ECU plug
thanks for the extra information
so its definitely fuel related as it responds with external fuel
but you've replaced the harness, ECM and the injectors?
and it has the correct fuel pressure?
can I ask what figure you saw for fuel pressure?
OK with the engine idling and the vacuum pipe connected you should have at least 30 psi so thats OK too
I would say the next thing to try is check that you're getting the 12V on the ECM fuse and check that the engine earth connections are clean and tight as there should be a couple and the small wire type that are secured onto the engine with ring terminals are the ones for the ECU