Patent Publication Number: US-2011056463-A1

Title: Carburetor control system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates mainly to a carburetor control system for a general-purpose engine, and relates especially to an improvement of a carburetor control system comprising: a governor device coupled to a throttle lever for opening and closing a throttle valve, the governor device opening the throttle valve when an operation of an engine is stopped, the governor device opening or closing the throttle valve in accordance with a rotational number of the engine when the engine is in operation; a choke return spring urging a choke lever for opening and closing a choke valve in a direction to close the choke valve; and an automatic choke device connected to the choke lever to open the choke valve in accordance with an increase in temperature of the engine. 
     2. Description of the Related Art 
     Such a carburetor throttle valve control system has already been known as disclosed in for example Japanese Patent Application Laid-open No. 2006-242143. 
     In such a conventional carburetor control system, the choke valve is controlled to a fully closed state by the automatic choke device when the engine is in a cold idling state. In addition, the carburetor is generally provided with a relief mechanism that opens the choke valve in accordance with an increase in the intake negative pressure generated downstream of the choke valve when the intake negative pressure reaches a predetermined value or more. Here, in the relief mechanism, it is desired that a set load of a relief spring is set as small as possible in order to facilitate the opening of the choke valve with the intake negative pressure, thereby preventing an excessive injection of fuel during cold idling of the engine. However, since the pulsation of the intake negative pressure is relatively strong when the engine is in an idling state, if the set load is set sufficiently small, the degree of opening of the choke valve becomes unstable due to the pulsation of the intake negative pressure. This being the situation, the reality is that the set load of the relief spring cannot be set sufficiently small. In this point, there is a room for improvement of the fuel efficiency of the engine. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of such circumstances, and aims to provide a carburetor control system that mechanically opens a choke valve to a predetermined intermediate degree of opening, when an engine is in a cold idling state, in operative connection with the closing of a throttle valve to a degree of opening for idling, so as to be capable of ensuring a fuel-efficient and stable idling state. 
     In order to achieve the object, according to a first feature of the present invention, there is provided a carburetor control system comprising: a governor device coupled to a throttle lever for opening and closing a throttle valve, the governor device opening the throttle valve when an operation of an engine is stopped, the governor device opening or closing the throttle valve in accordance with a rotational number of the engine when the engine is in operation; a choke return spring urging a choke lever for opening and closing a choke valve in a direction to close the choke valve; and an automatic choke device connected to the choke lever to open the choke valve in accordance with an increase in temperature of the engine, characterized in that the throttle lever is configured to pivot the choke lever, during a cold operation of the engine, to a position where the choke valve is at an intermediate degree of opening in operative connection with the throttle lever being pivoted by the governor device to a position where the throttle valve is at a degree of opening for idling or a position in a vicinity thereof. 
     According to the first feature of the present invention, after cold start of the engine, in operative connection with the throttle lever being pivoted to the position where the throttle valve is at the degree of opening for idling or a position in the vicinity thereof by the operation of the governor device, the choke lever is pivoted to the position where the choke valve is at the intermediate degree of opening. Thus, without being affected by the pulsation of the intake negative pressure, the amount of fuel injecting out from the fuel nozzle can stably be optimized by stabilizing the position where the choke valve is at the intermediate degree of opening, and a fuel-efficient and stable cold idling state of the engine can be ensured. 
     According to a second feature of the present invention, in addition to the first feature, the throttle lever is provided with a drive arm that pivots the choke lever, during the cold operation of the engine, to the position where the choke valve is at the intermediate degree of opening in response to the throttle lever being pivoted to the position where the throttle valve is at the degree of opening for idling or the position in the vicinity thereof, and the drive arm  24  has an engagement surface formed therein, the engagement surface receiving frictional resistance due to an urging force of the choke return spring from the choke lever in a pivoting direction when the drive arm pivots the choke lever to the position where the choke valve is at the intermediate degree of opening. 
     According to the second feature of the present invention, when the engine is in a cold idling state, the drive arm  24  of the throttle lever can pivot the choke lever to the position where the choke valve is at the intermediate degree of opening or a position in the vicinity thereof. Also, using the repulsive force of the choke return spring generated at this time, fluctuations of the throttle lever can be suppressed. Thus, the degree of opening of the throttle valve for idling can be stabilized to further stabilize the cold idling state of the engine. 
     Further, according to a third feature of the present invention, in addition to the second feature, the engagement surface is formed from an arc surface about a pivoting center of the throttle lever. 
     According to the third feature of the present invention, the repulsive force of the choke return spring can be used more effectively to reduce fluctuations of the throttle lever. Moreover, even when the throttle lever slightly pivots in the vicinity of the position where the throttle valve is at the degree of opening for idling, the choke valve can be maintained at the intermediate degree of opening. 
     According to a fourth feature of the present invention, in addition to the first feature, the choke lever is coupled to an outer end part of a valve shaft of the choke valve, the outer end part protruding upward from a carburetor body, the throttle lever is configured to pivot the choke lever, during the cold operation of the engine, to the position where the choke valve is at the intermediate degree of opening in operative connection with the throttle lever being pivoted by the governor device to the position where the throttle valve is at the degree of opening for idling or the position in the vicinity thereof, and an intermediate member pivotable relative to the choke lever is disposed between the choke lever and an upper surface of the carburetor body. 
     According to the fourth feature of the present invention, after cold start of the engine, in operative connection with the throttle lever being pivoted to the position where the throttle valve is at the degree of opening for idling or a position in the vicinity thereof by the operation of the governor device, the choke lever is pivoted to the position where the choke valve is at the intermediate degree of opening. Thus, without being affected by the pulsation of the intake negative pressure, the amount of fuel injecting out from the fuel nozzle can stably be optimized by stabilizing the position where the choke valve is at the intermediate degree of opening, and a fuel-efficient and stable cold idling state of the engine can be ensured. 
     Further, the intermediate member pivotable relative to the choke lever is disposed between the choke lever and the upper surface of the carburetor body, and the choke lever is separated from the upper surface of the carburetor body. Accordingly, in a cold condition, even if the freezing of rainwater or wash water adhering to the upper surface of the carburetor body causes the fixing of the intermediate member to the body, the pivoting of the choke lever is not inhibited. Thus, the throttle lever can reliably pivot the choke lever against the urging force of the choke return spring, thereby opening the choke valve to the intermediate degree of opening. 
     According to a fifth feature of the present invention, in addition to the fourth feature, the intermediate member is formed from a collar rotatably fitted to the valve shaft of the choke valve. 
     According to the fifth feature of the present invention, the collar as the intermediate member is supported between the carburetor body and the choke lever only by being fitted to the outer periphery of the valve shaft of the choke valve. Accordingly, the valve shaft of the choke valve is utilized for the supporting to eliminate the need for a special supporting member. 
     The above and other objects, characteristics and advantages of the present invention will be clear from detailed descriptions which will be provided below for the preferred embodiment while referring to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a general-purpose engine according to the present invention in which part is longitudinally sectioned; 
         FIG. 2  is an enlarged view of an essential part in  FIG. 1 ; 
         FIG. 3  is a sectional view taken along line  3 - 3  in  FIG. 2 ; 
         FIG. 4  is a sectional view taken along line  4 - 4  in  FIG. 2 ; 
         FIG. 5  is a sectional view taken along line  5 - 5  in  FIG. 2 ; 
         FIG. 6  is a sectional view taken along line  6 - 6  in  FIG. 2 ; 
         FIG. 7  is a diagram, corresponding to  FIG. 6 , for explaining the operation of an automatic choke device; 
         FIG. 8  is an another diagram for explaining the operation of the automatic choke device; 
         FIG. 9  is a vet another diagram for explaining the operation of the automatic choke device; 
         FIG. 10  is an enlarged view of a temperature sensitive section of the automatic choke device in  FIG. 6 ; 
         FIG. 11  is a diagram, corresponding to  FIG. 10 , for explaining the operation; 
         FIG. 12  is a schematic side view of a governor device; 
         FIG. 13  is an enlarged sectional view taken along line  13 - 13  in  FIG. 4 ; 
         FIG. 14  is a sectional view taken along line  14 - 14  in  FIG. 13 ; and 
         FIG.15  is a sectional view taken along line  15 - 15  in  FIG. 13 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described below based on the attached drawings. 
     Firstly, in  FIG. 1  to  FIG. 3 , reference symbol E denotes a four-cycle engine, which is a power source for various types of work machine. This engine E includes: a crankcase  2  supporting a vertically disposed crankshaft  1 ; a cylinder block  3  projecting horizontally from the crankcase  2  and having a cylinder bore  3   a;  and a cylinder head  4  formed integrally with an outer end part of the cylinder block  3 . The cylinder head  4  includes: an intake port  6   i  and an exhaust port  6   e  which are opened and closed by an intake valve  7   i  and an exhaust valve  7   e,  respectively; and a valve operating chamber  9  housing a valve operating mechanism  8  for operating the intake valve  7   i  and the exhaust valve  7   e.  A head cover  5  for closing the valve operating chamber  9  is joined to an end face of the cylinder head  4 . 
     Outer ends of the intake port  6   i  and the exhaust port  6   e  open respectively on one side face and another side face, which face opposite directions to each other, of the cylinder head  4 . A carburetor body Ca of a carburetor C is joined via a plurality of through bolts  12  to the one side face with a plate-shaped heat-insulating member  10  sandwiched therebetween. The carburetor body Ca includes an intake path  11  communicating with the intake port  6   i  of the cylinder head  4 . The heat-insulating member  10  is made of a thermosetting synthetic resin such as a phenol resin having excellent thermal insulation, thereby suppressing heat conduction from the engine E to the carburetor C. An exhaust muffler  14  communicating with the exhaust port  6   e  is mounted on said another side face of the cylinder head  4 . A fuel tank  17  and a recoil type starter  15  are disposed in an upper part of the engine E. Here, in  FIG. 1 , reference numeral  16  denotes a spark plug screwed into the cylinder head  4 . 
     As shown in  FIG. 2  and  FIG. 4 , the carburetor C is attached with an air cleaner  13  that communicates with the upstream side of the intake path  11 . 
     The carburetor C includes a choke valve  19  of a butterfly type for opening and closing an upstream portion of the intake path  11 , a throttle valve  20  also of a butterfly type for opening and closing the downstream side thereof, and a fuel nozzle (not illustrated) that opens in an opening between the two valves  19  and  20 . Both valve shafts  19   a  and  20   a  respectively of the choke valve  19  and the throttle valve  20  are rotatably supported in the carburetor body Ca in a vertical position. 
     As shown in  FIGS. 4 and 13  to  15 , the valve shaft  19   a  of the choke valve  19  is disposed offset to one side from the center line of the intake path  11 . The choke valve  19  is inclined with respect to the center line of the intake path  11  so that, when the choke valve  19  is in a fully closed state, a side of the choke valve  19  having a larger radius is on the downstream side of the intake path  11  relative to a side thereof having a smaller radius. A choke lever  22  is mounted on an outer end part of the valve shaft  19   a  projecting upward from the carburetor body Ca. This choke lever  22  includes a closed-bottom cylindrical hub  22   a  rotatably fitted to the valve shaft  19   a,  and a lever arm  22   b  integrally provided on one side surface of this hub  22   a  in a protruding manner. Between a lower end surface of this hub  22   a  and an upper surface of the carburetor body Ca, a collar  61  is rotatably fitted to the outer peripheral surface of the valve shaft  19   a  so as to be allowed to rotating relative to the hub  22   a.  On the inner side of the hub  22   a , a pair of stopper protrusions  62  and  62 ′ are formed that are spaced at regular intervals in the peripheral direction of the hub  22   a.  A relief lever  63  pivotable only between these stopper protrusions  62  and  62 ′ is fixed to the valve shaft  19   a,  and a relief spring  64  urging this relief lever  63  to bring the relief lever  63  into contact with one stopper protrusion  62  located on a side to close the choke valve  19  is provided between the hub  22   a  and the relief lever  63 . The relief lever  63  and the relief spring  64  constitute a relief mechanism  60 . 
     On the outer periphery of a lower portion of the hub  22   a,  a pair of stopper walls  65  and  65 ′ are formed which are spaced at intervals in the peripheral direction. A stopper pin  66  placed between these stopper walls  65  and  65 ′ is provided on the outer surface of the carburetor body Ca in a protruding manner. 
     Thus, a closed position C of the choke valve  19  is defined by one stopper wall  65  coming into contact with the stopper pin  66 , and an open position O of the choke valve  31  is defined by the other stopper wall  65 ′ coming into contact with the stopper pin  66 . The choke lever  22  is urged toward the closed position C by the urging force of a choke return spring  21 . The choke return spring  21  is formed from a torsion coil spring. A coil portion  21   a  thereof is disposed so as to surround the collar  61 . One end portion  21   b  thereof is latched onto a latching pin  68  provided on the upper surface of the carburetor body Ca in a protruding manner, and the other end portion  21   c  thereof is latched onto the stopper wall  65  of the hub  22   a.    
     When the choke valve  19  is fully closed or at a small degree of opening, if the intake negative pressure of the engine reaches a predetermined value or more, the difference between a rotational moment due to the intake negative pressure acting on the side of the choke valve  19  having a larger radius and a rotational moment due to the intake negative pressure acting on the side of the choke valve  19  having a smaller radius overcomes a rotational moment due to the relief spring  64 , and increases the degree of opening of the choke valve  19 . The increase of the degree of opening is restricted by the relief lever  63  coming into contact with the other stopper protrusion  62 ′. 
     To the lever arm  22   b  of the choke lever  22 , an automatic choke device A is integrally connected, the automatic choke device automatically controlling the degree of opening of the choke valve  19  in accordance with a change in temperature of the engine E. This automatic choke device A will be described with reference to  FIGS. 2 to 11 . 
     Referring firstly to  FIGS. 2 to 6 , the automatic choke device A includes: a temperature sensitive section  25  that receives heat from the cylinder head  4  of the engine E, in particular the area around the intake port  6   i ; and an output section  26  that provides connection between the temperature sensitive section  25  and the lever arm  22   b , and transmits a heat-receiving operation of the temperature sensitive section  25  to the choke lever  22  as a movement in a direction to open the choke valve  19 . The temperature sensitive section  25  has a cylindrical housing  30  disposed in a housing chamber  27  formed in the cylinder head  4  from a peripheral wall  4   a  of the intake port  6   i  and a surrounding wall  4   b  rising up from an upper part of the peripheral wall  4   a  (see  FIGS. 2 and 3 ). The housing chamber  27  has one end that opens, as an inlet, on one side face of the cylinder head  4  in the same manner as for the intake port  6   i,  and an end part on the opposite side facing the center of the cylinder head  4  is closed. Furthermore, one side of the housing chamber  27  is appropriately opened while taking into consideration the moldability of the surrounding wall  4   b  and the assemblability of the temperature sensitive section  25 . 
     The housing  30  includes: a cup-shaped first portion  30   a  made of a metal having excellent thermal conductivity such as Al and having a base part  30   a ′; and a cylindrical second portion  30   b  that is made of a synthetic resin having excellent thermal insulation such as a phenol resin and that is fitted in a telescoping manner into an open end of the first portion  30   a  and connected thereto via a screw  45  (see  FIG. 2 ). The second portion  30   b  is connected integrally to the heat-insulating member  10  disposed between the cylinder head  4  and the carburetor C. Therefore, the housing  30  is mounted on the cylinder head  4  without providing a member used exclusively for mounting. 
     The first portion  30   a  is disposed so that the base part  30   a ′ faces the interior side of the housing chamber  27 , that is, a central part (high temperature part) of the cylinder head  4 , and the base part  30   a ′ and the peripheral wall are in contact with an inner face of the housing chamber  27  or face it across a very small gap. The second portion  30   b  is disposed on the inlet side of the housing chamber  27 , that is, the side away from the center of the cylinder head  4 . 
     As shown in  FIG. 10 , the temperature sensitive section  25  includes: a bottomed movable cylinder  31  made of a metal having excellent thermal conductivity such as Al; a guide member  32  joined by crimping to an open end of the movable cylinder  31 ; a rod-shaped fixed piston  33  slidably supported in the guide member  32  and running therethrough; an elastic bag  34  having an open end held in a liquid-tight manner between the movable cylinder  31  and the guide member  32  while covering the fixed piston  33  within the movable cylinder  31 ; and wax  35  enclosed in the interior of the movable cylinder  31  so as to cover the elastic bag  34 . The movable cylinder  31  is slidably fitted within the first portion  30   a  of the housing  30  in a state in which the outer end of the fixed piston  33  abuts against an inner face of the base part  30 a′ of the first portion  30   a  of the housing  30 . 
     When the wax  35  is heated, it expands and compresses the elastic bag  34  so as to squeeze it, and consequently attempts to push the fixed piston  33  outside the guide member  32 , but since the fixed piston  33  having the outer end abutting against the inner face of the base part  30   a ′ of the first portion  30   a  is immovable, by virtue of the reaction thereof, the movable cylinder  31  advances within the first portion  30   a  in the direction of arrow F (see  FIG. 11 ), that is, in a direction in which it moves away from the, base part  30   a′.    
     The half of the outer peripheral face of the movable cylinder  31  on the side opposite to the guide member  32  has a small diameter; a distance collar  36  is fitted around this smaller diameter part  31   a,  and a coil-shaped return spring  38  is provided under compression between the heat-insulating member  10  and a retainer  37  abutting against the distance collar  36 , the return spring  38  urging, via the distance collar  36 , the movable cylinder  31  toward the outer end of the fixed piston  33 . Therefore, the retainer  37  is held between the distance collar  36  and the return spring  38 . 
     As shown in  FIGS. 5 and 6 , the output section  26  includes: a rod  43  running through the heat-insulating member  10  and coupling one end part  43   a  to the retainer  37 ; and first and second levers  42  supported, via a common pivot  40 , on opposite sides of a bracket  10   a  formed integrally with the heat-insulating member  10  so that they can pivot individually, another end part  43   b  of the rod  43  bent into an L shape being connected to the first lever  41 , and it is arranged so that the first lever  41  pivots in the direction of arrow R in  FIG. 6  as a result of axial movement of the rod  43  accompanying forward movement F of the movable cylinder  31 . Coupling of the rod  43  to the retainer  37  is achieved by holding an enlarged end part  43   a  at one end of the rod  43  between the retainer  37  and an end face of the movable cylinder  31 . 
     The first and second levers  41  and  42  have abutment parts  41   a  and  42   a  that separably abut against each other along the pivotal direction of the two, and these abutment parts  41  a and  42   a  move away from each other when the first lever  41  pivots in the direction of the arrow R relative to the second lever  42 . The first and second levers  41  and  42  are provided with spring latching parts  41  b and  42   b,  and opposite ends of a coupling spring  44  are latched onto these spring latching parts  41   b  and  42   b,  the coupling spring  44  urging the two levers  41  and  42  in a direction in which the abutment parts  41   a  and  42   a  abut against each other. 
     Formed integrally with the second lever  42  is an operating arm  42   c  that operatively faces a follower pin  22   c  of the lever arm  22   b.  When the second lever  42  pivots in the direction of the arrow R, the operating arm  42   c  makes the choke lever  22  pivot in a direction to open the choke valve  19 . 
     In  FIG. 12 , a governor device G for automatically controlling opening and closing of the throttle valve  20  is explained. A throttle lever  23  is secured to an outer end part of the valve shaft  20   a  of the throttle valve  20 . A governor lever  52  is secured to the outer end of a rotating support shaft  51  supported on the engine E. A long arm portion  52   a  of the governor lever  52  is coupled to the throttle lever  23  via a link  53 . Furthermore, coupled via a governor spring  54  to the governor lever  52  is an output control lever  56  that is supported on the engine E, etc. and can pivot through a range from an idling position to a full load position. The governor spring  54  always urges the throttle valve  20  in the opening direction, and its spring load is increased and decreased by pivoting the output control lever  56  from the idling position to the full load position, or vice versa. 
     Further, an output shaft  55   a  of a known centrifugal governor  55  driven by the crankshaft  1  of the engine E is connected to a short arm portion  52   b  of the governor lever  52 . The output of the centrifugal governor  55 , which increases in response to an increase in the rotational number of the engine E, acts on the short arm portion  52   b  in a direction to close the throttle valve  20 . 
     Therefore, in a state in which running of the engine E is stopped, the throttle lever  23  is held by means of a set load of the governor spring  54  at a position in which the throttle valve  20  is fully opened, but during running of the engine E, the degree of opening of the throttle valve  20  is automatically controlled by the balance between the moment of the governor lever  52  due to the output of the centrifugal governor  55  and the moment of the governor lever  52  due to the set load of the governor spring  54 . 
     Moreover, as shown in  FIGS. 4 and 7 , a drive arm  24  protruding toward the choke lever  22  is formed integrally with the throttle lever  23 . This drive arm  24  is configured to drive the lever arm  22   b  of the choke lever  22  to open the choke valve  19  from the fully closed position to a predetermined intermediate degree of opening when the throttle valve  20  is closed to the degree of opening for idling or a degree of opening close thereto. Further, the surface  24   a  of the drive arm  24  that engages the lever arm  22   b  is formed in the shape of an arc about the valve shaft  20   a  of the throttle valve  20  so that, when the throttle valve  20  is closed to the degree of opening for idling or a degree of opening close thereto, a tip end portion of the lever arm  22   b  is pushed toward the valve shaft  20   a  of the throttle valve  20  by the urging force of the choke return spring  21  to come into contact with the engagement surface  24   a.    
     Next, the operation of this embodiment is explained. 
     In a state in which the engine E is cold or stopped, as shown in  FIG. 10 , since the wax  35  in the temperature sensitive section  25  is in a contracted state, the movable cylinder  31  is held at a retracted position in proximity to the base part  30   a ′ of the first portion  30   a  of the housing  30  by means of the resilient force of the return spring  38 . Accompanying this, as shown in  FIG. 6 , the operating arm  42   c  of the second lever  42  of the output section  26  is held at a position spaced from the lever arm  22   b  of the choke lever  22 , and therefore the choke lever  22  is held at a position in which the choke valve  19  is closed by means of the urging force of the choke return spring  21 . 
     On the other hand, the throttle valve  20  is held at a fully open position by the governor spring  54  since the centrifugal governor  55  is in an inoperative state (see  FIGS. 4 and 6 ). If at this time the output control lever  56  is set at the idling position, the load of the governor spring  54  is set to a minimum. 
     Therefore, in order to start the engine E, if the recoil starter  15  is operated so as to crank the crankshaft  1 , a large negative pressure is generated in the intake path  11  downstream of the choke valve  19  in the carburetor C, a relatively large amount of fuel spurts out from the fuel nozzle, which opens at this position, to make a gas mixture formed in the intake path  11  rich, thereby smoothly starting the engine E. 
     When the engine E is started, the centrifugal governor  55  generates an output corresponding to the rotational number of the crankshaft  1 , so that the governor lever  52  pivots in a direction in which the moment of the governor lever  52  due to the above output balances the moment of the governor lever  52  due to the spring force of the governor spring  54 . As shown in  FIG. 7 , the throttle valve  20  is closed to the degree of opening for idling by the throttle lever  23 . In this process of closing the throttle valve  20 , the drive arm  24  integrated with the throttle lever  23  pivots the lever arm  22   b  of the choke lever  22  in a direction to open the choke valve  19  against the urging force of the choke return spring  21 . Thus, when the choke valve  19  is opened to a predetermined intermediate degree of opening (e.g., 30°), the tip end portion of the lever arm  22   b  comes into contact with the arc-shaped engagement surface  24   a  of the drive arm  24 . As a result, as much air as needed to idle the engine E can be smoothly taken into the intake path  11  without depending on the operation of the relief mechanism  60 , and the choke valve  19  can be prevented from fluctuating due to the pulsation of the intake negative pressure of the engine. It is possible to stabilize the injection of fuel while suppressing an excessive injection of fuel from the fuel nozzle. Accordingly, the gas mixture supplied to the engine can be regulated to have an air/fuel ratio appropriate for a cold idling state, so that the fuel efficiency of the engine E can be improved. 
     Also, in this state, the lever arm  22   b  is pushed by the repulsive force of the choke return spring  21  to come into contact with the engagement surface  24   a  of the drive arm  24  and, even fluctuations of the throttle lever  23  is suppressed by the frictional force generated therebetween. Accordingly, a more stabilized cold idling state of the engine can be obtained. 
     In particular, as described previously, the engagement surface  24   a  is formed from an arc surface about the valve shaft  20   a  of the throttle valve  20 , and the tip end portion of the lever arm  22   b  is pushed toward the valve shaft  20   a  of the throttle valve  20  by the urging force of the choke return spring  21  to come into contact with the engagement surface  24   a.  Accordingly, fluctuations of the throttle lever  23  can effectively be suppressed by the frictional force generated in the contact portion. Also, even when the throttle lever  23  slightly pivots in the vicinity of the position where the throttle valve  20  is at the degree of opening for idling, the choke valve  19  can be maintained at the aforementioned intermediate degree of opening. 
     Incidentally, in a cold condition, rainwater or wash water staying on the upper surface of the carburetor body Ca may freeze. In such a case, if a lower surface of the choke lever  22  is in direct coiitact with the upper surface of the carburetor body Ca, the choke lever  22  would be fixed to the carburetor body Ca due to the above-described freezing, and the pivoting of the choke lever  22  by the drive arm  24  of the throttle lever  23  would be inhibited. However, the collar  61  pivotable relative to the hub  22   a  is disposed between the hub  22   a  of the choke lever  22  and the upper surface of the carburetor body Ca to prevent the choke lever  22  from coming into direct contact with the carburetor body Ca. Accordingly, even if rainwater or wash water adhering to the upper surface of the carburetor body Ca freezes, only the collar  61  is fixed, and the choke lever  22  can be prevented from being fixed. Thus, after the starting up of the engine E, in the process of closing the throttle valve  20  to the degree of opening for idling by the throttle lever  23 , the drive arm  24  can reliably pivot the choke lever  22  against the urging force of the choke return spring  21 , thus opening the choke valve  19  to a predetermined intermediate degree of opening. Also, the collar  61  is supported between the carburetor body Ca and the choke lever  22  only by being fitted to the outer periphery of the valve shaft  19   a  of the choke valve  19 . Accordingly, the valve shaft  19   a  of the choke valve  19  is utilized for the supporting to eliminate the need for a special supporting member. 
     Next, when the output control lever  56  is pivoted from the idling position to an appropriate load position in order to impose a load of a work machine, etc. on the engine E, the load on the governor spring  54  increases accordingly, so that the degree of opening of the throttle valve  20  when the load on the governor spring  54  and the output of the centrifugal governor  55  are in balance therefore increases. When the throttle valve  20  increases beyond the degree of opening for idling, the drive arm  24  of the throttle lever  23  is released from the pressing force of the lever arm  22   b  of the choke lever  22 . However, since the rotational number of the engine increases to become stabilized, the operation of the centrifugal governor  55  also becomes stabilized, and the throttle lever  23  is also prevented from fluctuating. 
     Moreover, an increase in the amount of intake air accompanying an increase in the degree of opening of the throttle valve  20  also stabilizes the intake negative pressure generated downstream of the intake path  11 . When the intake negative pressure exceeds a predetermined value, the choke valve  19  is opened until the difference between the rotational moment due to the intake negative pressure acting on the side of the choke valve  19  having a larger rotational radius and the rotational moment due to the intake negative pressure acting on the side of the choke valve  19  having a smaller rotational radius balances the rotational moment due to the relief spring  64  within the choke lever  22 . Therefore, it is possible to suppress an excessive injection of fuel from the fuel nozzle and prevent the gas mixture formed in the intake path  11  from becoming too rich, thus guaranteeing good warm-up operating conditions. At this stage, since the intake negative pressure becomes relatively stable with an increase in the amount of intake air as described previously, the choke valve  19  never fluctuate, though the choke valve  19  is not forced to open by the drive arm  24  of the throttle lever  23 . 
     When the temperature of the cylinder head  4  increases accompanying progress in the warming up of the engine E, the temperature sensitive section  25  within the housing chamber  27  in the proximity of the intake port  6   i  is heated via an inner wall of the housing chamber  27 ; as described above, the reaction to the elastic bag  34  being constricted to push out the fixed piston  33  due to thermal expansion of the wax  35  within the movable cylinder  31 , makes the movable cylinder  31  move forward in the direction of the arrow F against the resilient force of the return spring  38 ; and this forward movement of the movable cylinder  31  pivots the first lever  41  via the rod  43  in the direction of the arrow R. Since this first lever  41  and the second lever  42  are initially in a coupled state in which the abutment parts  41   a  and  42   a  abut against each other due to the urging force of the coupling spring  44 , as shown in  FIG. 7 , the second lever  42  also pivots integrally with the first lever  41 , and the operating arm  42   c  makes the follower pin  22   c,  that is, the choke lever  22 , pivot against the urging force of the choke return spring  21  in a direction that opens the choke valve  19 . Therefore, since the degree of opening of the choke valve  19  increases in response to an increase in the temperature of the housing chamber  27 , the negative pressure above the fuel nozzle within the intake path  11  is decreased accompanying progress in the warming up of the engine E, the amount of fuel spurting out from the fuel nozzle is decreased, and the air/fuel ratio of the gas mixture formed in the intake path  11  can be appropriately corrected. About time when the warming up of the engine E is completed, the temperature of the interior of the housing chamber  27  is sufficiently high, and as shown in  FIG. 8 , the choke valve  19  is controlled so as to be in a fully open state. 
     As hereinbefore described, when the choke valve  19  is opened by the choke lever  22 , the choke lever  22  moves away from the drive arm  24  of the throttle lever  23  as shown in  FIG. 8 , and the two levers  22  and  23  do not interfere with each other. Therefore, if the output control lever  56  is returned to the idling position to control the load of the governor spring  54  at a minimum after the warming up is completed, the throttle lever  23  can be pivoted to the degree of opening for idling of the throttle valve  20  by means of the output of the centrifugal governor  55  without the interference of the choke lever  22 . 
     When the temperature of the cylinder head  4  further increases and the temperature of the housing chamber  27  increases, the wax  35  further thermally expands, and the movable cylinder  31  moves fonvard excessively to thus further pivot the first lever  41  in the direction of the arrow R via the rod  43 . However, since further pivoting of the second lever  42  is inhibited by the choke lever  22  at the fully open position, as shown in  FIG. 9 , the first lever  41  alone pivots in the direction of the arrow R while stretching the coupling spring  44 , and the abutment part  41   a  of the first lever  41  moves away from the abutment part  42   a  of the second lever  42 . Therefore, an over stroke operation of the movable cylinder  31  of the temperature sensitive section  25  is absorbed by the stretching of the coupling spring  44 . This means that each section from the automatic choke device A to the choke valve  19  is not applied by a load that is higher than the set load of the coupling spring  44 , thereby avoiding the occurrence of excessive stress in each section to secure the durability of each section. Moreover, since the first and second levers  41  and  42 , which can pivot relative to each other, are mounted on the bracket  10   a  via the common pivot  40 , it is possible to reduce the number of components of the output section  26 , thus simplifying the structure. 
     When running of the engine E is subsequently stopped, as long as a high temperature state of the engine E continues, the interior of the housing chamber  27  is also kept in a high temperature state, and therefore the temperature sensitive section  25  maintains a state in which the movable cylinder  31  is moved forward, thus maintaining the choke valve  19  in an open state via the output section  26 . Accordingly, when the engine E in a high temperature state is restarted, it is possible to secure an open state of the choke valve  19 , prevent the gas mixture from becoming too rich, and achieve good restarting properties. 
     After running of the engine E is stopped, when it becomes cool, the movable cylinder  31  retracts in the temperature sensitive section  25  as a result of thermal contraction of the wax  35  and the action of the return spring  38 . Therefore, the output section  26  allows pivoting of the choke lever  22  by means of the choke return spring  21  in a direction that closes the choke valve  19 . 
     The present invention is not limited to the above-mentioned embodiment, and various design modifications can be made thereto without departing from the gist thereof. For example, instead of the centrifugal governor device G, an electronic governor device or other type of governor device may be provided. Instead of the wax-type pe automatic choke device A, an electrothermal automatic choke device or other type of automatic choke device may be provided.