Weber Marine Carb on 360 la engine?

The passage must be clogged. You can try carb cleaner in the fitting or remove the carb, flip it over, put your finger in the primary bore next to the buttery fly, put your palm over the welch plug, then blow a couple psi of compressed air with a rubber tip nozzle into the outside fitting. Clean out the large hole then put the welch plug back in. The surface is covered by the gasket when installed.