Carburetor for internal combustion engines with controlled ignition, especially designed to equip vehicles, of the type wherein - in order to obtain a fine and continuous regulation of the feed mixture strength - a controlled depression is produced into the top of the constant level float chamber. For this purpose, the top of the float chamber is connected to external atmosphere, both downstream of the air register throttle and in correspondence of the rim of said throttle, the position of said air register throttle being determined by an electric motor controlled by sensors detecting the combustion state in the engine.