[flyerncle]

What a question,Simple terms, the piston's going up and down in the engine sucks fuel from the needle in the carbs.
To get the best performance it should be matched as closely as possible to the inlet ports on the head,not always possible but make sure the manifold flange and gasket are as closely matched to port size as possible, this way the fuel/air is not hitting the edges of the head and causing turbulence and in effect wasting power.

It is possible with care to open the ports up but only to match the gasket,but be carefull. Take off casting marks and polish as much as poss.

There is nothing mega scientific in it and the airspeed across the venturi in the center of the carb rises and causes the fuel to be sucked out through the jet.

Hope this makes a little sense.