Idle operation of Solex 4a1 carburetor
by Patryk Rebisz

The fuel enters the fuel bowl through a filter (37) and a float valve (35). At idle the throttle plates (160) are closed and very little air enters through the carb throats. The fuel (really, mixture of fuel and air bubbles) is sucked through tiny orifice below throttle plate into the engine. The amount of that fuel is controlled through the mixture screw (16) AND ease of that fuel being sucked in (fuel level in the bowl).

If you were to follow the fuel's trek, the fuel is sucked through the main jet (79) at the bottom of the bowl then bypasses main mixing tube (162) entering idle mixing duck. In there, the fuel is mixed with air that enters through idle air jet (78). From this point the fuel is emulsified. It then travels through fuel shut off valve (21), past transition holes, idle mixture screw and into main intake throats.

Notice the gaskets (in green) through which the fuel has to travel. If the carb is warped, it develops internal leaks where the fuel and air is exposed to other forces thud impeding its smooth travel.