US3852391A

US3852391A - Carburetor with deceleration circuit

Info

Publication number: US3852391A
Application number: US00233602A
Authority: US
Inventors: T Hisatomi; K Sasaki
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1971-03-11
Filing date: 1972-03-10
Publication date: 1974-12-03
Anticipated expiration: 1991-12-03
Also published as: GB1379952A; JPS502775B1; FR2129546A5; DE2211698B2; DE2211698A1; DE2211698C3

Abstract

A carburetor for an automotive internal combustion engine which is capable of eliminating unburned engine exhaust gases emitted to the atmosphere during decelerating operation of the engine by supplying the engine with an air-fuel mixture of optimum amount and mixture ratio that are specifically suited for decelerating condition of the engine. The carburetor includes a by-pass passage having an inlet port communicating with a carburetor induction passage upstream of a venturi and an outlet port communicating with said carburetor induction passage downstream of a throttle valve, an air jet provided in the by-pass passage, a fuel jet provided in the by-pass passage downstream of the air jet, and a control unit for controlling the flow of air-fuel mixture passing through the by-pass passage in response to an intake manifold vacuum prevailing in the carburetor induction passage downstream of the throttle valve.

Description

States atent isatomi et a].

[ 1 Dec. 3, 1974 [541 CARBURETOR WITH DECELERATION FOREIGN PATENTS OR APPLICATIONS CIRCUIT l,9l6,639 10/1970 Germany t. ital/DIG l) [75] Inventors: Tak'ashi Hisatomi; Kenichi Sasaki,

both of Yokohama Japzm Primary Examiner-Tim R. Miles [73] Assignee: Nissan Motor Company, Limited,

Yokohama City, Japan 7 ABSTRACT [22] Filed: 1972 A carburetor foran automotive internal combustion [21] APP] No: 233,602 engine which is capable of eliminating unburned en- I gine exhaust gases emitted to the atmosphere during decelerating operationof the engine by supplying the Foreign Application i y Data engine with an air-fuel mixture of optimum amount Mar. 11, 197! Japan 46-13272 and mixture ratio that are specifically suited for decelj i erating condition of the engine. The carburetor in- [52] U.S. CL. 261/69 R, 26l/DIG. l9, 26l/DIG. 21, cludes a by-pass passage having an inlet port commu- 123/97 B nicating with a carburetor induction passage upstream [51] Int. Cl. F02m 7/12 of a venturi and an outlet port communicating with [58] Field of Search... 26l/DlG. 19, 69 R, DIG. 55; said carburetor induction passage downstream of a 123/97 B throttle valve,,an air jet provided in the by-pass pas- I I sage, a fuel jet provided in the by-pass passage down- [56] References Cited I stream of the air jet, and a control unit for controlling UNITED STATES PATENTS the flow of air-fuel mixture passing through the byl 265 I94 5/1918 Mock et a1 ZGHDIGSS pass passage in response to an intake manifold vac- 2824'726 H1958. Dietrich 'gj: zm/DlG: 19 uum prevailing in the carburetor induction passage 3:503:59: 3/l970 00w 26l/DIG. l9 downstream Ofthe throttle Valve- 3,575,386 4 1971" Boyd 261/DIG1 l9 3,677,526 7/1972 Pierlot ..-.....'26I/I)Io. 19 1 Clam" 3 Drawmg F'gures lg 54 5 2O 58 FF- 9 56 44 I a a /46 28 l E IOO "74 6 This invention relates in general to a carburetor for an internal combustion engine of a motor vehicle and more particularly to a carburetor adapted to reduce the unburned content of exhaust gases emitted from the engine during deceleration.

During deceleration of a motor vehicle with the throttle valve substantially fully closed and the engine driven by the motor vehicle, the flow of air-fuel mixture is practically shut off at the throttle valve to cause the vacuum in the intake manifold to rise abruptly. An additional mixture circuit, called the idling and slowrunning mixture circuit, is therefore provided to supply an air-fuel mixture to the engine during deceleration in an amount and mixture ratio that are predetermined. The amount and mixture ratio of the air-fuel mixture are usually predetermined to suit the idling operation of the engine. The engine is thus supplied during idling and deceleration with an air-fuel mixture which is predetermined to enablethe engine to operate satisfactorily during the idling, not the decelerating operation. This is reflected by reduced combustion efficiency and misfiring in the combustion chambers of the engine during deceleration so that unburned exhaust gases are emitted from the engine to the atmosphere causing serious air pollution especially in urban areas.

To solve the vehicular air-pollution problem to be traced to the unburned toxic content of engine exhaust gases, the present invention contemplates to provide a carburetor which is constructed and arranged in such a manner that an air-fuel mixture of optimum amount and mixture ratio is fed during deceleration to the engine through a mixture circuit which is independent of the idling and slow-running mixture circuit. The independent mixture circuit is herein referred to as deceleration circuit" because it is operable during deceleration. i

It is a principal object of the present invention to provide an improved carburetor for an internal combustion engine of a motor vehicle whichcarbur'etor is capable of supplying the engine with an air-fuel mixture of an amount and mixture ratio that are specifically suited for the deceleration in addition to the air-fuel mixture fed to the engine through the existing idling and slow-running mixture circuit.

Anotherobject of the present invention is to provide an improved carburetor for an internal combustion engine of a motor vehicle which carburetor is simple in construction and economical to manufacture.

A further object of the present invention is to provide an improved carburetor for an internalcombustion engine ofa motor vehiclewhich carburetor has a deceleration circuit adapted to supply the engine with an airfuel mixture of optimum amount and mixture ratio during deceleration to effect complete combustion of the air-fuel mixture supplied to the engine for thereby eliminating unburned hydrocarbon and carbon monoxide contents in engine exhaust gases emitted to the atmosphere.

Inv general these objects are achieved in the present invention by incorporating a deceleration circuit in a carburetor of an internal combustion engine. The deceleration circuit includes a by-pass passage having an inlet port communicating with a'carburetor induction passage upstream of the venturi and an outlet port communicating with the carburetor induction passage downstream of the throttle valve. An air jet is provided in the by-pass passage to control the flow rate of air passing therethrough. A fuel jet is also provided in the by-pass passage downstream of the air jet, the fuel jet leading from a fuel supply by-pass passage communicating with a float chamber. To control the flow of airfuel mixture passing through the .by-pass passage, a control unit is incorporated in the carburetor. The con- .trol unit includes a housing having first and second cavities which act as first and second vacuum chambers, respectively. The housing also has a spring seat which intervenes between the first and second cavities and which is formedwitha central aperture therein. A cap member is attached to one endof the housing, a first diaphragm member being interposed between the cap member and the one end of the housing. The cap member has formed with a central opening vented to the atmosphere for a first atmospheric chamber defined between the inner surface of the cap member and the first diaphragm member. A control valve element is connected to the first diaphragm member and extends through the central aperture formed in the spring seat to open or close the same. A passage is provided which has one end communicating with the first vacuum chamber and the other end communicating with the carburetor induction passage downstream of the throttle valve. The first diaphragm member is biased by a first coiled compression spring in a'direction to cause the control valveelement to open the central aperture for providing communication between the first and second vacuum chambers. The first coiled compression spring engages at one end thereof with the disc assembly attached to the first diaphragm member and at the other end thereof with the spring seat. The control unit further includes a second diaphragm member which is interposed between the other end of the housing and the carburetor body. The carburetor body is formed with a cavity which constitutes a second atmospheric chamber in association with the second diaphragm member. The second atmospheric chamber communicates with the carburetor induction passage upstream of the venturi. An adjusting valve element is connected to and movable with the second diaphragm member, the adjusting valve element extending through an opening formed in said carburetor body into the by-pass passage to open or close the same. A second compression spring is disposed in the second vacuum chamber for biasing the second diaphragm member in a direction to cause the adjusting valve element to close the by-pass passage. During deceleration of the engine with the throttle valve substantially fully closed and the engine driven by the motor vehicle, the intake manifild vac uum prevailing in the carburetor. induction passage downstream of the throttle valve abnormally increases. This intake manifold vacuum is supplied through the passage to the first vacuum chamber. When, in this instance, the intake manifold vacuum in the firstvacuum chamber reaches a predetermined value, the pressure difference on opposite sides of the first diaphragm member will be sufficient to overcome the biasing force of the first compression spring and the first diaphragm member will move in a directionto cause the control valve element to open the central aperture. Upon opening of the central aperture, the second diaphragm member in the second vacuum chamber will besubjected to the'intake manifold vacuum, and the pressure difference on opposite sides of the second diaphragm member-will effect movement of the second diaphragm member in a direction to cause the adjusting valve to open the by-pass passage. When the by-pass passage is open, the air flowing across the fuel jet carries a suction effect which causes a flow of fuel from that float chamber into the by-pass passage. Thus, the additional air is mixed with fuel resulting in an air-fuel mixture having an appropriate mixture ratio for eliminating the un burned exhaust gases during deceleration of the engine.-

These and other objects and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sectional view of a preferred embodiment of a carburetor according to the present invention;

FIG. 2 is a view illustrating a typical example of variations of the intake manifold vacuumobtained with the use of the carburetor shown in FIG. 1; and

FIG. 3 is a view illustrating the relationship between intake manifold vacuum and the stroke .of the valve element forming part of the control unit of the carburetor shown in FIG. 1. v

Referring now to the drawings and more particularly to FlG. 1, there is shown in section a carburetor implementing the present invention. which carburetor is suited for use with .an internal combustion engine of a motor vehicle. The carburetor, which is generally designated by reference numeral 10, includes a carburetor body 100 which is provided with a carburetor induction passage 12 leading from an air cleaner (not shown) to an intake manifold (not shown) of the engine, a throttle valve 14 disposed in the carburetor induction passage 12, a venturi l6"formed in the carburetor induction passage 12 upstream of the throttle valve 14, a main mixture circuit 18 having a main mixture nozzle 20 opening into the venturi 16, an idling and slow-running mixture circuit 22 having a slow-running port 24 located at a position closely adjacent to the periphery of the throttle valve 14 when it is substantially fully closed and an idling port 26 located downstream of the throttle valve 14, and a float chamber 28 for containing therein liquid fuel 30. The throttle valve 14 is mounted on an angularly adjustable shaft Ma in the carburetor induction passage 32 and is herein shown as substantially fully closed to effect idling or deceleration of the engine. The main mixture circuit 18, through which an air-fuel mixture is supplied to the carburetor induction passage 12 for relatively heavy load operation such as acceleration or high speed driving of the engine, communicates with the float chamber 28 through a first orifice 32. The idling and slow-running mixture circuit 22 for idling or light load operation communicates with j the float chamber 28 through, in addition to the first orifice 32, asecond orifice 34 and opens to the carburetor induction passage through the slow-running port 24'and the idling port 26.

Under normal idling conditions with the throttle valve 14 substantially fully closed, as shown, manifold vacuum will draw sufficient idling fuel through the

orifices

32 and 34 to maintain the engine idling at a low speed. However, under decelerating conditions. e.g. the motor vehicle moving at relatively high speeds with the the throttle valve 14 substantially fully closed, a high vacuum develops in the carburetor induction passage l2 downstream of the throttle valve 14 and in the engine intake manifold due to and in accordance with buretor 10 with a view to supplying the engine with an the reciprocating movement of the pistons of the engine. The high vacuum thus created downstream of the throttle valve 14 acts on the slow-running port 24 and the idling port 26 causing liquid fuel 30 in the float chamber 28 to flow through the idling and slowrunning mixture circuit 22 in excess of that required to maintain the engine in an idling condition. This excess liquid fuel is then passed through the exhaust system of the engine to the atmosphere in an unburned state since there is insufficient air for complete combustion. To eliminate this drawback a deceleration circuit according to the present invention is provided in the carair-fuel mixture of optimum amount and mixture ratio that are specifically suited for decelerating conditions.

The deceleration mixture circuit, which is generally indicated at 36, includes a by-pass passage 38 having an inlet port 38a communicating with the carburetor induction passage 12 upstream of the venturi 16 and an outlet port 38b communicating with the carburetor induction passage 12 downstream of the throttle valve 14. The by-pass passage 38 is provided adjacent its upstream end with an air jet 40 to control the flow rate of air passing therethrough and a fuel jet 42 provided in a fuel supply by-passage 44 opening to the by-pass passage 38 downstream of the air jet- 40. The by-pass passage 44 leads from the float chamber 28 through a port 440. The effectiveareas of the air jet 40 and the fuel jet 42 are so calibrated as to provide an air-fuel mixture of proper mixture ratio that is specifically suited for maintaining the engine at decelerating conditions to minimize the air pollutant content in the engine exhaust gases.

To control the flow of air-fuel mixture passing through the by-pass passage 38, there is further provided in the carburetor 10 a control unit 46 which is responsive to the vacuum prevailing in the carburetor induction passage 12 downstreamof the throttle valve 14. The control unit 46 includes a housing 48 having first and second cavities 50 and 52 which act as first and second vacuum chambers, respectively, in a manner as will be hereinafter described in detail. The housing 48 also has a spring seat 54 which intervenes between the first and second cavities 50 and 52, the spring seat 54 being formed with a central aperture 56 which interconnects the second vacuum chamber 52 with the first vacuum chamber 50.

The right hand end of the housing 48 is closed by a cap member 58, a suitable first diaphragm member 60 being interposed between the cap member 58 and the right hand end of the housing 48. The cap member 58 is attached to the housing 48 by a suitablefastener means, though not shown. The cap member 58 is formed with a central opening 62 which constitutes an atmospheric vent for the first atmospheric chamber 64 formed between the right hand surface of the first diaphragm member and the inner surface of the cap member 58.

To control the degree of communication between the first and second vacuum chambers 50 and 52, a control valve element 66 is operatively disposed in the central aperture 56. The control valve element 66has one end formed witha frusto-conical valve head 66a and the other end connected to the first diaphragm member 60 by means of a disc assembly 68.

As seen from FIG. 1, a coiled compression spring 70 is provided between the spring seat 54 and the first diaphragm member 60 for biasing the first diaphragm member 66 rightwardly as viewed in FIG. I, that is, in a direction to cause the valve head 66a of the control valve element 66 to seat on the valve seat 540 formed on the spring seat 54 for thereby interrupting communication between the first and second chambers 50 and 52. One end of the coil spring 70 engages with the right hand side of the spring seat 56 while the other end thereof engages with a left hand side disc of the disc assembly 68.

As seen in FIG. I, the first cavity or the vacuum chamber 56' has a port Stla communicating with the carburetor induction passage 12 downstream of the throttle valve 14 through a passage 72. When the vacuum chamber 56 is subjected to a vacuum of a predetermined magnitude through the port 50a, the pressure difference acting on opposite sides of the diaphragm member 66 will be sufficient to overcome the resisting force of the calibrated coiled compression spring 70 and, hence, the diaphragm member 60 will move leftwardly of the drawing, as viewed in FIG. I, to a position where the valve head 66a of the control valve element 66 is unseated from the valve seat 54a to interconnect the first vacuum chamber 56 with the second vacuum chamber 52.

The left hand end of the housing 48 is attached to and maintained in assembled relation with the carburetor bodyat lltla by a suitable fastener means, not shown. Between these parts, a second diaphragm member 74 is interposed which has attached thereto a disc assembly 76, to which an adjusting valve element 78 is secured. The adjusting valve element 78 extends through an opening 66 formed in the carburetor body We into the by-pass passage 38 to open and close the same. The opening 86 is so sized as to permit smooth movement of the adjusting valve element 76 therethrough. A second coiled compression spring 82 is disposed between the spring seat 54 and the second diaphragm member 7 1 for biasing the second diaphragm member Ml leftwardly, as viewed in FIG. I, to a position where the adjusting valve element 76 closes the by-pass passage 38. One end of the coiled compression spring 62 engages with the left hand side of the spring seat 54 while the other end of the coiled compression spring fil engages with the right hand side disc of thedisc assembly 76.

The second vacuum chamber 52 is formed with a port 52a which interconnects the vacuum chamber 52 with the atmosphere or the carburetor induction passage llZ upstream ofthe venturi 16 through a calibrated air bleed M. The calibrated air bleed M- for the chamber 52 prevents the trapping of vacuum in chamber 52 when the control valve element 66 closes the central aperture 56 so that the second diaphragm member 74 will respond quickly and in a follow-up manner to the movements of the first diaphragm member 66. In other words, the vacuum chamber 52 is bled to the atmo sphere or the carburetor induction passage through the air bleed 8d sothat unless the control valve element 66 closes the central aperture 56, the coiled compression spring 62 is operative to 'move the diaphragm member 74 leftwardly, as viewed in FIG. l, and thereby position the adjusting valve element 76m close the by-pass passage 36.

As seen in FIG. it, the carburetor body lltla is formed with a cavity which defines a second atmospheric chamber 86 in association with the second diaphragm member 74. This chamber 66 communicates with the carburetor induction passage 12 upstream of the venturi 16 through a passage 88 so that a pressure difference developes across the second diaphragm member 74. In the illustrated embodiment, the passage 86 is shown as opening to the carburetor induction passage 12 but may be constituted to open to the atmosphere. Indicated at 96 is an orifice which interconnects the second atmospheric chamber with the passage 72 communicating with the carburetor induction passage l2 downstream of the throttle valve 114. The orifice permits the air-fuel mixture drawn to the chamber 86 through the gap formed between the opening 86 and the adjusting valve element 78 to flow into the passage 72 through which the air-fuel mixture is supplied to the engine thereby eliminating waste of the fuel.

During normal running conditions of an internal combustion engine equipped with a carburetor according to the present invention, fuel is supplied through either the main fuel nozzle 20 or the idling and

slowrunning ports

26 and 26 in a conventional manner. Moreover, the air-fuel ratio of the air-fuel mixture during normal running operation is not in any way affected by the carburetor disclosed herein. However, during deceleration of the engine underconditions where the motor vehicle drives the engine with the throttle valve substantially fully closed, the intalke manifold vacuum reaches an abnormally high value. In some cases the intake manifold vacuum reach a value as high as 650 mm of Hg. The resisting spring load on the first diaphragm member 60 is adjusted so that this diaphragm member is not actuated until the intake manifold vacuum exceeds a value which is created during normal running operation, for instance, 550 mm of Hg. When the intake manifold vacuum reaches a value of 550 mm of Hg, the first diaphragm member 60 will move leftwardly of the drawing due to the pressure difference on opposite sides thereof and the control valve element 66 will open the central aperture 56. When the valve head 66a of the control valve element 66 is unseated from the valve seat 540, the intake manifold vacuum prevailing in the first vacuum chamber Stl is communicated to the second vacuum chamber 52. when the second vacuum chamber 52 is subjected to an intake manifold vacuum even of a relatively low value, the second diaphragm member 74 will move rightwardly of the drawing against the force of the compression spring 82 so as to displace the adjusting valve element 78 to open the by-pass passage 36. Thus, air will be permitted to flow through the air jet 60 into the downstream part of the by-pass passage 36 due to the high intake manifold vacuum prevailing in the carburetor induction passage downstream of the throttle valve 14!. Air flowing across the fuel jet t2 creates an aspirating effect due to its high velocity so s to result in fuel being drawn from the float chamber 26 through the fuel supply by-pass passage 44 to the passage 38. The fuel admitted to the passage 36 is mixed with air flowing therethrough to form an air-fuel mixture which is supplied directly to the intake manifold of the engine through the carburetor induction passage 112. The air-fuel mixture supplied by the deceleration circuit 36 mixes with the idling air-fuel mixture supplied by the carburetor idling and slow running mixture circuit 22 and forms a combustible mixture which is distributed by the intake manifold to the several engine cylinders and purges the exhaust gases therefrom. In this manner, the unburned content in the engine exhaust gases during deceleration is eliminated.

The control unit 46 associated with the deceleration circuit 36 operates intermittently during deceleration with a substantially fully closed throttle valve, since as soon as the intake manifold vacuum decreases below a predetermined value, the first diaphragm member 60 will move rightwardly of the drawing by the force of the coiled compression spring '70 to cause the valve head 66a of the control valve element 66 to be seated on the valve seat 54a to interrupt communication between the first and second vacuum chambers 50 and 52. When this occurs, the second vacuum chamber 52 reaches an the variations in the intake manifold vacuum of the'engine equipped with a conventional carburetor whereas curve B indicates the variations in the intake manifold vacuum of the engine equipped with the carburetor according to the present invention. a

FIG. 3 illustrates the relationship between the intake manifold vacuum and the stroke of the adjusting valve element 78 of the control unit 46 shown in FIG. 1. In FIG. 3, a solid line C shows the rate of variation of the intake manifold vacuum of the engine equipped with a carburetor having a conventional control unit. A dotted line D indicates the rate of variation of the intake manifold vacuum of the engine equipped with a carburetor having incorporated therein the control unit forming part of the invention.

It will now be understood from the foregoing description that the carburetor implementing the present invention is capable of supplying the engine with an airfuel mixture of optimum amount and mixture ratio that are specifically suited for deceleration of the engine to eliminate the unburned contents in the engine exhaust gases emitted to the atmosphere.

It will also be appreciated that the carburetor of the present invention is simple in construction and economical to manufacture.

While one embodiment of the present invention has been herein shown and described in association with the accompanying with the drawings, it is intended to merely for exemplifying the carburetor according to the present invention and it should be understood that the, carburetor of the present invention may be modified in many respects without departing from the spirit and scope ofthe present invention which are defined in the appended claims.

What is claimed is:

1. A carburetor for an internal combustion engine comprising a carburetor body, a carburetor induction passage formed in said carburetor body, a venturi in said carburetor induction passage, a throttle valve operatively disposed in said carburetor induction passage, a main mixture circuit opening into said venturi in said carburetor induction passage for supplying an air-fuel mixture to said carburetor induction passage for highspeed and acceleration operations of said engine, an idling and slow-running mixture circuit opening into said carburetor induction passage downstream of said throttle valve for supplying an air-fuel mixture to said carburetor induction passage for low-speed and deceleration operations of said engine, a float chamber which supplies fuel to said two circuits, a by-pass passage having an inlet port communicating with said carburetor induction passage upstream of said venturi and an outlet port comunicating with said carburetor induction passage downstream of said throttle valve, an air jet provided in said bypass passage to control the flow rate of air passing therethrough, a fuel jet provided in a fuel supply by-pass passage opening to the first mentioned by-pass passage downstream of said air jet and leading directly from said float chamber, and a control unit for controlling the flow of air-fuel mixture passing through the first mentioned by-pass passage in response to the vacuum prevailing in said carburetor induction passage downstream of said throttle valve, said control unit including a housing having first and second vacuum chambers and a spring seat intervening therebetween said spring seat having formed therin acentral aperture, said second vacuum chamber provided with a port communicating with said carburetor induction passage upstream of said venturi through a calibrated air bleed, a passage having one end communicating with said carburetor induction passage downstream of said throttle valve and the other end communicating with said first vacuum chamber, a cap member attached to one end of said housing and having an opening vented to the atmosphere, a first diaphragm member interposed between said one end of said housing and said cap member, a first atmosphere chamber formed between said first diaphragm member and the inner surface of said cap member, a control valve element connected to said first diaphragm member and extending through said central aperture formed in said spring seat to control communication between said first and second vacuum chambers, a first compression spring disposed in said first vacuum chamber for biasing said first diaphragm member in a direction to cause .said control valve element to close said central aperture of said spring seat for thereby interrupting communication between said first and second vacuum chambers, a second diaphragm member interposed between the other end of said housing and said carburetor body, a second atmospheric chamber defined in a cavity formed in said carburetor body and said second diaphragm member, said second atmospheric chamber communicating with said carburetor induction passage upstream of said venturi, means defining an orifice between said second atmospheric chamber and said passage having one end communicating with said induction passage downstream of said throttle valve, an adjusting valve element connected to said second diaphragm member and extending through an opening formed in said carburetor body into said by-pass passage to open and close said by-pass passage, and a second compression spring disposed in said second vacuum chamber for biasing said second diaphragm member in a direction to cause said adjusting valve element to close said by-pass passage, said first vacuum chamber being subjected to the vacuum prevailing in said carburetor induction passage downstream of said throt tle valve, whereby when the vacuum in said first vacuum chamber exceeds a predetermined value during deceleration operation of said engine, said first dia phragm member is moved against the force of said first compression spring in a direction tocause said control valve element to open said central aperture formed in W the vacuum to act on and cause said second diaphragm member to move against the force of said second compression spring in a direction to cause said adjusting said spring seat for interconnecting said first vacuum Valve element to open i bypass Passagechamber with said second vacuum chamber to permit

Claims

1. A carburetor for an internal combustion engine comprising a carburetor body, a carburetor induction passage formed in said carburetor body, a venturi in said carburetor induction passage, a throttle valve operatively disposed in said carburetor induction passage, a main mixture circuit opening into said venturi in said carburetor induction passage for supplying an air-fuel mixture to said carburetor induction passage for highspeed and acceleration operations of said engine, an idling and slow-running mixture circuit opening into said carburetor induction passage downstream of said throttle valve for supplying an air-fuel mixture to said carburetor induction passage for lowspeed and deceleration operations of said engine, a float chamber which supplies fuel to said two circuits, a by-pass passage having an inlet port communicating with said carburetor induction passage upstream of said venturi and an outlet port comunicating with said carburetor induction passage downstream of said throttle valve, an air jet provided in said by-pass passage to control the flow rate of air passing therethrough, a fuel jet provided in a fuel supply by-pass passage opening to the first mentioned by-pass passage downstream of said air jet and leading directly from said float chamber, and a control unit for controlling the flow of air-fuel mixture passing through the first mentioned by-pass passage in response to the vacuum prevailing in said carburetor induction passage downstream of said throttle valve, said control unit including a housing having first and second vacuum chambers and a spring seat intervening therebetween said spring seat having formed therin a central aperture, said second vacuum chamber provided with a port communicating with said carburetor induction passage upstream of said venturi through a calibrated air bleed, a passage having one end communicating with said carburetor induction passage downstream of said throttle valve and the other end communicating with said first vacuum chamber, a cap member attached to one end of said housing and having an opening vented to the atmosphere, a first diaphragm member interposed between said one end of said housing and said cap member, a first atmosphere chamber formed between said first diaphragm member and the inner surface of said cap member, a control valve element connected to said first diaphragm member and extending through said central aperture formed in said spring seat to control communication between said first and second vacuum chambers, a first compression spring disposed in said first vacuum chamber for biasing said first diaphragm member in a direction to cause said control valve element to close said central aperture of said spring seat for thereby interrupting communication between said first and second vacuum chambers, a second diaphragm member interposed between the other end of said housing and said carburetor body, a second atmospheric chamber defined in a cavity formed in said carburetor body and said second diaphragm member, said second atmospheric chamber communicating with said carburetor induction passage upstream of said venturi, means defining an orifice between said second atmospheric chamber and said passage having one end communicating with said induction passage downstream of said throttle valve, an adjusting valve element connected to said second diaphragm member and extending through an opening formed in said carburetor body into said by-pass passage to open and close said by-pass passage, and a second compression spring disposed in said second vacuum chamber for biasing said second diaphragm member in a direction to cause said adjusting valve element to close said by-pass passage, said first vacuum chamber being subjected to the vacuum prevailing in said carburetor induction passage downstream of said throttle valve, whereby when the vacuum in said first vacuum chamber exceeds a predetermined value during deceleration operation of said engine, said first diaphragm member is moved against the force of said first compression spring in a direction to cause said control valve element to open said central aperture formed in said spring seat for interconnecting said first vacuum chamber with said second vacuum chamber to permit the vacuum to act on and cause said second diaphragm member to move against the force of said second compression spring in a direction to cause said adjusting valve element to open said by-pass passage.