Classic vapor lock and percolated fuel.
The heat from the exhaust basically is boiling the gas in the fuel bowls ‘percolating ‘ and lines. So there a just bubbles or gases and the fuel pump can’t push it to the carbs either.
If you notice 260s had an auxiliary electric pump to help pushing cool fuel from the tank.
Also you will notice early z’s have a heat reflecting insulation on the fuel lines.
I bypassed the stock ‘puller’ pump mounted on the head running off the the cam.
I use an old quiet cheap Mazda rx7 pump below the gas tank to push the fuel.
It has worked with SUs and Mikunis. With the heat of headers.
Additionally I added the fan that a lot of ZXs had that mounts to the engine and blows air over the cam cover onto the intake and exhaust. A heat shield above the exhaust manifold helps too
The oxen are slow, but the Earth is patient.
1970 240Z, 1972 240Z, 2003 350Z