ORIGINAL: Dr1Driver
The intake stroke of the piston creates a negative pressure under the carb which pulls in air. The air moves past the fuel spraybar, pulling fuel into the carb by the venturi effect. The amount of fuel is regulated by the needle valves. Muffler pressure helps the engine draw fuel at idle, when the negative pressure is the least.
Dr.1
But isn't muffler backpressure highest at full speed and almost non existent at idle? I think that with muffler pressure helping deliver fuel, the main needle needs to be closed a few clicks at full speed. Because the main needle is closed more, there can be more vacuum at idle at the spraybar for a correct fuel/air mix. On an air bleed carb, this means the air bleed can be closed a little more for a better fuel draw at idle.