the embodiment of a fuel supply system according to the present invention will be described with reference to accompanying drawings . fig1 shows a manifold in general comprising a manifold entrance 1 , a riser portion 2 disposed perpendicularly to the manifold entrance 1 , an exhaust passage 4 provided so as to come in contact with a floor surface of the riser portion 2 , and a plurality of branch passages 40a , 40b , . . . each extending through the outside of the riser portion 2 and communicating with each cylinder ( not shown ), respectively . as seen from fig2 the manifold entrance 1 comprises a throttle chamber 7 in which two throttle vaves 6a and 6b are provided , and an injection chamber 9 in which a fuel injection valve 8 is provided , which will be described later in more detail . a primary passage 10 and a secondary passage 11 are formed by partitioning a passage consisting of both chambers 7 and 9 . the throttle valves 6a and 6b are disposed at the respective entrances 10a and 11a of the passages 10 and 11 , respectively . on the other hand , at the exit portions 10b and 11b , the passages 10 and 11 are formed ring - shaped wherein the secondary passage 11 is concentrically disposed with respect to the outer circumference of the primary passage 10 . it is here noted that the primary passage 10 has a cross - sectional area of the passage sufficient to obtain a predetermined air flow mostly in the region of a low load condition . the secondary passage 11 also has a cross sectional area of the passage sufficient to obtain a predetermined air flow in the region of medium and high loads . with respect to the primary throttle valve 6a and the secondary throttle valve 6b , it is necessary to present a throttle opening characteristic as shown in fig5 . for this purpose , a lever 13 of the throttle valve 6a and a lever 14 of the throttle valve 6b are associated as shown in fig4 so as to enable a combined operation via a pin 16 and a slot 17 . as long as the locus of the pin 16 becomes approximately coincident with the curved central axis of the slot 17 , the opening of the throttle valve 6b is small in comparison with that of the throttle valve 6a . when the opening of the throttle valve 6a exceeds , for instance , about 60 °, the throttle valve 6b is rapidly opened . therefore , a relatively small but constant amount of air is introduced into the primary passage 10 from a low load to a high load condition , while a large amount of air is introduced into the secondary passage 11 mainly in the region of medium and high loads . the exit portion 10b of the primary passage 10 is formed with a venturi 19 as is best shown in fig1 thereby squeezing air flow to step up air flow velocity . the injection nozzle 8 of the fuel injection valve 8 is disposed so that it is directed to the central passage of the venturi 19 . thus , fuel injection in the direction of the corrugated radiation plate 3 of the riser portion 2 through a venturi throat 19a is made possible . a flow regulating plate 22 for eliminating the directionability of the primary air flow is provided outside the injection nozzle portion 8a . also , in order to eliminate the directionability of the secondary air flow , another flow regulating plate 23 is provided at the exit 11b of the secondary passage 11 . thus , these flow regulating plates make it possible to increase the uniformity of the distribution of the mixture with respect to each cylinder . the fuel which is regulated by a pressure regulator 24 is supplied into the fuel injection valve 8 through a passage 25 . the pressure regulator 24 controls the pressure of the fuel injection so as keep a differential pressure constant between intake vacuum pressure downstream of the throttle valve and the fuel supply pressure . an air regulator 26 is provided so as to bypass air through the throttle valve 6a of the primary passage 10 and then introduce it upstream of the injection nozzle portion 8a . the air regulator 26 is provided with a thermo - responsive valve 28 for opening or closing a bypass passage 27 , thereby making it possible to open the passage 27 in response to the temperature while the engine is cooling . after a predetermined time passes from the starting of the engine , the air regulator 26 becomes operative to automatically close the passage 27 by responding to the heat by means of , for example , a heater ( not shown ) assembled therein . in order to constantly maintain a predetermined air flow at the time of idling , there is provided an idle air guide passage 29 communicating with the upstream side of the valve 28 provided in the bypass passage 27 . the passage 29 is provided at the throat portion 19a of the venturi 19 with a plurality of branch openings 30 radially disposed therein . thus , it is possible to promote the fuel atomization at the time of idling . reference is now made to the operation of the fuel supply system according to the present invention . when the engine is under a low load condition , that is , the opening of the throttle valve 6a is , for example , less than 30 °, the opening of the secondary throttle valve 6a is extremely small . accordingly , air being introduced into the engine passes mostly through the primary passage 10 . the primary air flow has the maximum value in flow speed when passing through the venturi portion 19 , and is conducted so as to collide against the corrugated radiation plate 3 of the riser portion 2 . the fuel is injected from the injection nozzle portion 8a of the fuel injection valve 8 in the same direction as the high speed air flow . accordingly , the fuel is atomized by the high speed air flow and is subject to the heat from the corrugated radiation plate 3 , so that fuel evaportion will be effectively promoted . the flow regulating plate 22 prevents the air flow from being partially scattered with respect to the branch passages 40a and 40b communicating with each cylinder , whereby the distribution of the mixture is uniformly effected . it is to be noted that , even in the region of low - load condition , there is a slight amount of air passing through the secondary passage 11 . therefore , the secondary air flows around the exit of the primary passage 10 perpendicularly projected , thereby moderately preventing the fuel from being attached to the inner wall surface of the passage . it should be noted that there is an extremely small amount of air flow , such as , for example , in an idling condition even in the region of a low load . in such a case , in comparison with air fuel flowing from the upstream side of the injection nozzle portion 8a , air flow being supplied from the idle air guide passage 29 communicating with the venturi 19 is more than that flowing from the upstream side of the injection nozzle portion 8a . as a result , although the overall air flow is little , it is possible to maintain the atomizing characteristic satisfactory owing to the colliding against the air flow being supplied through the passage 29 . for instance , at the starting time when the engine is cooling , the air necessary for effecting idling operation of the engine is introduced into the primary passage 10 . accordingly , fuel atomization is effectively promoted at the time of not only the idling operation , but also warming - up driving , thereby making it possible to maintain stabilization at the time of the idling operation and reducing the warming - up time . on the other hand , in the region of medium and high loads on the engine , the ratio of the intake air flow being supplied through the secondary throttle valve 6b in addition to the air flow being supplied through the primary throttle valve 6a gradually increases . air being supplied from the secondary passage 11 and air being supplied from the primary passage 10 are joined together at their respective exits 10b and 11b . this secondary air flow forms an annular - shaped high speed air layer around the venturi portion 19 in which the primary air flows . as a result , the injection fuel riding on the primary air flow is guided by the annular - shaped high speed air layer in such a manner so as not to become attached to the inner wall of the secondary passage 11 and is effectively atomized , thereby maintaining satisfactory fuel atomization efficiency even in the region of medium and high loads . furthermore , in the region of medium and high loads , almost all of the air is conducted into the secondary passage 11 having the cross sectional area of the passage sufficiently larger than that of the primary passage 10 , thereby increasing the amount of the mixture with good response during acceleration , without increasing intake passage resistance . the fuel supply system according to the present invention makes it possible to atomize the fuel in such a manner so as to allow the fuel to ride on a high speed air flow from the region of a low load to that of a high load condition . further , the fuel supply device according to the present invention is constituted so as to reduce the amount of the fuel attached to the inner wall surface of the passage and so as not to present a specified direction of an air flow mixture . accordingly , although fuel injection is effected with the spi method , a very excellent atomizing characteristic and mixture distribution characteristic are obtained . furthermore , the fuel supply device according to the present invention also makes it possible to remarkably improve the acceleration responsiveness and exhaust performance . it is to be understood that modifications and variations of the embodiments of the invention disclosed herein may be resorted to without departing from the spirit of the invention and the scope of the appended claims .