Some carbs had a choke mechanism that relied on engine heat to pull open the choke blade. The center port you're referring to was the source of the heat.
Porting is something done to the intake, and exhaust ports in the cylinder head. It enlarges the ports allowing more air/fuel mixture to flow into the combustion chamber, making more power. An engine...
If you take a valve cover off, the rocker arm that lines up with an intake runner, is intake, and the ones that line up with exhjaust ports, are exhaust.