I'm not 1200% sure what revision of HS would be on a 79gt if the float lid has 2 pipes the one that isn't connected to the fuel line is the vent, if there's only one pipe the vent is hiding behind a little steel cover plate on the sticky up bit the fuel line goes in to. I guess you've checked and adjusted the float valve?
It's an SU HS4 carb, Eff. So the vent is behind the baffle plate? That should be clear now, I reckon.
What's this about the float valve? What have I missed?