Abstract:
A rotary throttle valve carburetor includes an air intake passage and a throttle valve bore formed in a carburetor body, a cam disposed in the throttle valve bore, and a throttle valve disposed in the throttle valve bore so that one end engages the cam and the other end is accessible from outside the throttle valve bore for operable connection to a throttle valve lever. The carburetor may also have a cable holder with a receiving cylinder for an outer tube of a throttle valve lever cable, an insert port for an inner wire of the cable and a slit between the receiving cylinder and insert port. The slit is smaller than an end of the inner wire to prevent that end from passing through the slit. The carburetor may further include a tamper resistant fuel adjustment bolt including an anti-turning body keyed to the carburetor body and splined to the adjustment bolt after calibration of the carburetor to prevent inadvertent rotation, or user tampering with the calibrated setting.

Description:
REFERENCE TO RELATED APPLICATIONS 
     Applicant claims priority of Japanese Patent Applications, Ser. No. 2002-161,710 filed Jun. 3, 2002; Ser. No. 2002-171,548 filed Jun. 12, 2002; and Ser. No. 2002-308,510 filed Oct. 23, 2002. 
     FIELD OF THE INVENTION 
     The present invention relates generally to a carburetor, and more particularly to a rotary throttle valve carburetor. 
     BACKGROUND OF THE INVENTION 
     In the conventional rotary throttle valve carburetor, moving the rotary throttle valve via a link or lever mechanism, an excessive or undesirably high force may be required to operate the throttle, which can be a limiting requirement in design. Further, a cam for causing the rotary throttle valve to move vertically is also provided between the carburetor body and a lever that actuates the rotary throttle valve. This limits the design freedom as far as shape of the cam and the type of mateiral that can be used. 
     In a fuel adjustment needle valve of a carburetor proposed in Japanese Patent Application No. 2000-045884, a head portion of an adjustment bolt is received in a vacant portion of a carburetor body, a plug or cap is pressed into the vacant portion and then caulked or the like so that the plug may not be removed, and the engine operator may not adjust the adjustment bolt arbitrarily. The above-described fuel adjustment needle valve of a carburetor poses a problem that the plug or cap is difficult to process and assemble. 
     As shown in  FIG. 16 , the conventional throttle valve operating mechanism for a rotary throttle carburetor includes a mounting flange  108  of a lid plate, a holding thread  103  cut in an outer peripheral portion at an end of a cable holder  102  provided with a hexagonal nut  101 , and an insert port  104  formed on a central portion at an extreme end of the cable holder  102 . For mounting a remote control cable, an end of the inner wire is drawn out of the insert port  104 , and the end of the inner wire is retained in a groove  106  of a swivel  105  of a throttle valve lever which engages an idle position bolt  109  when the throttle valve is in its idle position. Then play in the cable is adjusted by rotating the nut  101  and its holding thread  103 , and further positioning is done by a lock nut  107 . However, it is difficult to mount and retain the inner wire in the groove  106  of the swivel  105  since the surrounding space is narrow and crowded, and the split groove  106  is small. Further, since the insert port  104  is larger in diameter than the inner wire, the outer tube can fall off from the cable holder  102  during assembly. 
     SUMMARY OF THE INVENTION 
     A rotary throttle valve carburetor includes an air intake passage and a throttle valve bore formed in a carburetor body, a cam disposed in the throttle valve bore, and a throttle valve disposed in the throttle valve bore so that one end engages the cam and the other end is accessible from outside the throttle valve bore for operable connection to a throttle valve lever. The carburetor may also have a cable holder with a receiving cylinder for an outer tube of a throttle valve lever cable, an insert port for an inner wire of the cable and a slot between the receiving cylinder and insert port. The slit is smaller than an end of the inner wire to prevent that end from passing through the slit. The carburetor may further include a tamper resistant fuel adjustment bolt including an anti-turning body keyed to the carburetor body and splined to the adjustment bolt after calibration of the carburetor to prevent inadvertent rotation, or user tampering with the calibrated setting. 
     In one form, a driven gear formed on the upper end of the throttle valve and a drive gear connected to a shaft on which a valve lever of the throttle valve is mounted are meshed with each other, and the gears are disposed in an encasing chamber sealed by a lid plate to prevent dust or the like from fouling a valve chamber and the rotary throttle valve. An accelerating pump may be provided, and may be driven by a cam on the lower surface of the drive gear is disposed between the rotary throttle valve and the shaft on which the valve lever of the throttle valve is mounted to permit the size of the carburetor to be minimized. 
     According to another embodiment of a carburetor, a fuel adjustment needle valve is secured in place to prevent tampering or inadvertent rotation of the valve. An antiturning body is held in a holding hole formed on a pump cover plate, and after setting the position of an adjustment bolt, a spline hole formed on the antiturning body is fitted on a spline shaft portion of the adjustment bolt. An antiturning piece formed on the antiturning body is received in a groove in the carburetor body to prevent turning of the antiturning body and the adjustment bolt. 
     According to another aspect of the invention, a remote control throttle cable holder includes a receiving cylinder for receiving an end of an outer tube of the remote control cable, and an insert port in communication with the receiving cylinder through a slit is provided on the cable holder whereby the end of the inner wire is engaged with the slit when an end of the inner wire is mounted on a swivel of a throttle valve lever. The outside diameter of the slit is smaller than that of the end of the inner wire, therefore the outer tube is held on the cable holder so as not to fall off, and thus the cable is easy to mount to the cable holder. 
     Some potential objects, features and advantages of the invention include providing a carburetor in which a rotary throttle valve can be rotated smoothly, which is small and compact in size, has high design freedom, provides a fuel adjustment needle valve for a carburetor which prevents user tampering with a calibrated setting, and is of reltaively simple design and economical and easy manufacture and assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments, appended claims and accompanying drawings in which: 
         FIG. 1  is a front sectional view of one presently preferred embodiment of a rotary throttle valve carburetor taken generally along line  1 — 1  of  FIG. 4 ; 
         FIG. 2  is a front sectional view of the rotary throttle valve carburetor taken generally along line  2 — 2  of  FIG. 4 ; 
         FIG. 3  is a plan view of the rotary throttle valve carburetor; 
         FIG. 4  is a plan sectional view taken generally along line  4 — 4  of  FIG. 1  of the rotary throttle valve carburetor; 
         FIG. 5  is a side sectional view of the rotary throttle valve carburetor; 
         FIG. 6  is a perspective view of a rotary throttle valve and an annular cam; 
         FIG. 7  is another perspective view of the rotary throttle valve and annular cam; 
         FIG. 8  is a partial front sectional view showing an antiturning body of a fuel adjustment needle valve splined in a head portion of an adjustment bolt; 
         FIG. 9  is a front sectional view showing a fuel adjustment needle valve in which the antiturning body is held in a holding hole of a pump cover plate; 
         FIG. 10  is a perspective view of an alternate embodiment of a carburetor provided with a throttle valve operating mechanism; 
         FIG. 11  is a perspective view of the carburetor of  FIG. 10  with a portion in section showing the throttle valve operating mechanism; 
         FIG. 12  is a plan view of the carburetor of  FIG. 10  showing the throttle valve operating mechanism; 
         FIG. 13  is a side view of a lid plate provided with a cable holder of the throttle valve operating mechanism; 
         FIG. 14  is a side sectional view of a rear portion of the cable holder; 
         FIG. 15  is a fragmentary perspective view showing an end portion of an inner wire to be mounted on the cable holder; and 
         FIG. 16  is a perspective view of a carburetor according to the prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIGS. 1 and 2 , a carburetor has a carburetor body  14  with an air intake passage  51  extending through the body. In assembly, the carburetor is disposed between an air cleaner, not shown, and a wall surrounding an intake port of the engine, preferably with an insulating plate between them. The body  14  may be fastened to the engine by a pair bolts extending through mounting holes  17 . A rotary throttle valve  15  having a throttle hole  18  is rotatably and axially slidably received in a cylindrical valve chamber  21  that is formed perpendicular to the air intake passage  51  of the carburetor body  14 . 
     An annular cam  22  is provided at the bottom of the valve chamber  21  and has a pair of peripheral cam surfaces  22   a  ( FIG. 7 ) with a height that varies along the circumferential extent of the cam surfaces  22   a , as shown in  FIGS. 6 and 7 . The cam  22  has a pair of pins  22   b  which are inserted into pin holes in the valve chamber  21 . Rod-like followers  15   a  carried by the throttle valve  15  extend radially and are engaged with the cam surfaces  22   a  so that rotation of the throttle valve  15  causes axial movement of the throttle valve  15 . However, other arrangements are possible. For example without limitation, the cam surfaces  22   a  may be formed in the bottom surface of the rotary throttle valve  15 , and the follower  15   a  may be provided on the bottom of the valve chamber  21 . 
     An encasing chamber  50  surrounded by a wall  57 , shown in  FIG. 4 , is formed in the upper portion of the carburetor body  14 . A driven gear (preferably a partial gear)  35  is formed on the rotary throttle valve  15 . A lid plate  4  is put over the upper end of the carburetor body  14 , and secured by a plurality of bolts  56  ( FIG. 3 ). A return spring  16  is interposed between the lid plate  4  and the rotary throttle valve  15  with one end of the spring  16  disposed in an annular groove formed on the upper end of the rotary throttle valve  15 , and secured to the rotary throttle valve  15 . The other end of the spring  16  is disposed in an annular groove formed on the lid plate  4 , and secured to the lid plate  4 . The rotary throttle valve  15  is axially biased against the cam  22  and yieldably rotatably biased to its idle position by the force of the return spring  16 . 
     One end of a fuel supply pipe  20  is secured to the carburetor body  14 , and the other end of the fuel supply pipe  20  is projected into the throttle hole  18  of the rotary throttle valve  15 . A threaded portion  19   a  ( FIG. 5 ) is engaged in a threaded hole of the upper end portion of the rotary throttle valve  15 , and a needle  19  connected to the threaded portion  19   a  is inserted into the fuel supply pipe  20  to adjust the effective opening or flow area of a fuel nozzle hole  20   a  ( FIG. 5 ) of the fuel supply pipe  20 . 
     As shown in  FIG. 4 , a drive gear (preferably a partial gear)  39  meshed with the driven gear  35  is connected to a hollow shaft  9 . The hollow shaft  9  is rotatably engaged with a support shaft  40  secured to the carburetor body  14  in parallel to the rotary throttle valve  15 . The hollow shaft  9  projects upward through an opening of the lid plate  4 , and is connected to an operating lever  8 . A spring  7  ( FIGS. 1 and 2 ) is wound about the upper end of the hollow shaft  9  and the upper and lower ends thereof are secured to the operating lever  8  and the lid plate  4 , respectively. The operating lever  8  is yieldably biased into contact with an end of an idle stop bolt  13  by the force of the spring  7 . The idle stop bolt  13  is threaded in a wall  12  of the lid plate  4 , an outer tube of a known remote control cable is connected to hollow mounting fittings  2  which are threadedly received on a projecting flange wall  3  of the lid plate  4 , whereas an inner wire fitted in the outer tube is connected to a swivel  10  of the operating lever  8 . 
     As shown in  FIG. 5 , the axial position of the needle  19  can be adjusted by a screw driver or the like inserted into a hole  5  provided in the lid plate  4  to adjust the extent to which the fuel nozzle hole  20   a  is open when the throttle valve  15  is in its idle position. A steel ball  6  is pressed into the regulating hole  5  to inhibit or prevent alteration of the idle setting. 
     As shown in  FIGS. 1 and 2 , a fuel pump  30  and a purge and priming pump  47  are provided on the carburetor body  14 . A fuel pump diaphragm  32  is trapped between the body  14  and a plate  44 . A pump chamber  31  is formed on one side of the diaphragm  32 , and a pulsation pressure chamber  33  for introducing a pulsating pressure signal of a crank chamber of the engine is formed on the other side of the diaphragm  32 . When the diaphragm  32  is vibrated or displaced, fuel in a fuel tank, not shown, is taken into the pump chamber  31  via an inlet pipe  36  and an intake valve (not shown), and is supplied to a fuel metering chamber  26  via a discharge valve (not shown) and an inlet valve (not shown) at a fuel inlet  23  of a constant pressure fuel supply mechanism  25 . 
     A bulb  48  of the suction pump  47  is secured on the body  14  by a retaining plate  45  having an opening through which a portion of the bulb  48  projects. The retaining plate  45  is fastened to the carburetor body  14  by a plurality of bolts  46 . A mushroom shape composite check valve  42  is connected to the plate  44  within a pump chamber  43  defined in part by the bulb  48 . When the bulb  48  is repeatedly pressed prior to the start of the engine, fuel vapor in the fuel metering chamber  26  pushes open a bevel portion of the composite check valve  42  and flows into the pump chamber  43 . Subsequent depression of the bulb  48  pushes open a flat central tube portion of the composite check valve  42  and returns the vapor, air and/or liquid fuel in the bulb  48  to the fuel tank via an outlet pipe  60  ( FIG. 5 ). 
     The constant pressure fuel supply mechanism  25  has a cover plate  29  connected to the carburetor body  14  by bolts  24  with a fuel metering diaphragm  27  therebetween. The fuel metering chamber  26  and an atmospheric chamber  28  are defined on opposite sides of the diaphragm  27 , respectively. Although not shown, a lever mechanism is disposed in the fuel metering chamber  26  and oscillates in response to the vertical movement of the diaphragm  27  and opens and closes an inlet valve (not shown) disposed on the fuel inlet  23 . A fuel outlet  61  of the fuel metering chamber  26  is communicated with the fuel supply pipe  20  via a check valve (not shown) and a fuel adjustment needle valve  34 . 
     As shown in  FIGS. 2 and 4 , an accelerating pump  54  is disposed between the valve chamber  21  and the support shaft  40 . That is, a cam groove  39   a  of which depth becomes gradually shallower in a peripheral or circumferential direction is formed in the lower surface of the drive gear  39 , and the upper end of a plunger  52  is engaged with the cam groove  39   a  by the force of a spring. The plunger  52  is received in a cylindrical bore  53  formed in the carburetor body  14 . The lower end of the bore  53  is communicated with the fuel supply pipe  20  via a fuel passage  55 . One end of the fuel passage  55  is closed by a ball plug  58 . When the engine is accelerated, the cam groove  39   a  becomes increasingly shallower with the rotation of the drive gear  39 , the plunger  52  is pushed down, and fuel in the bore  53  is supplied to the fuel supply pipe  20  providing additional fuel to support engine acceleration. The fuel metering chamber  26  is communicated with the outlet passage  61 , a check valve, fuel adjustment needle valve  34 , passage  71  and the fuel supply pipe  20 , as shown in  FIG. 8 . 
     On the carburetor body  14  an air adjustment needle valve  62  is provided for adjusting the quantity of air in a bypass passage suitable for an increase of fuel when the engine starts in cold ambient weather. Further,  FIGS. 1 and 2  are synthesized from a front view and a plan view by CAD, and the fuel adjustment needle valve  34  is different from the actual shape and is shown bent. 
     As described above, according to the present invention, since the driven gear  35  formed on the rotary throttle valve  15  and the drive gear  39  connected to the hollow shaft  9  are meshed with each other, and the gears  35  and  39  are disposed in the encasing chamber  50  closed by the lid plate  4 , it is possible to completely prevent outside dust or the like from entering and fouling the valve chamber  21 , and since the operating lever  8  for operating the rotary throttle valve  15  is connected to the hollow shaft  9 , the total height of the carburetor can be reduced. 
     The idle position and the fully opened position of the rotary throttle valve  15  are controlled by the idle stop bolt  13  and the projecting wall  3  of the lid plate  4 , respectively, against which the operating lever  8  impinges. The accelerating pump  54  driven by the cam groove  39   a  on the lower surface of the drive gear  39  is disposed in a raised portion of the extra thick wall between the valve chamber  21  and the hollow shaft  9  to thereby minimize the size of the carburetor. 
     The hole  5  is provided coaxial with the needle  19  on the lid plate  4 . The threaded portion  19   a  of the needle  19  is turned by a screw driver or the like from within the hole  5  to adjust an opening degree of the fuel nozzle hole  20   a , and thereafter, the steel ball  6  is pressed into the hole  5 . Therefore, the valve chamber  21  is sealed from outside, and the seal force is received by the carburetor body  14 . Thus, the above feature does not impart adverse influence such as deformation to the sliding part of the rotary throttle valve  15 , and the steel ball  6  pressed into the regulating hole  5  prevents adjustment of the needle  19  setting to control the idle fuel and air mixture for improved exhaust gas control. 
     Since the cam body of the accelerating pump  54  is provided on the lower surface of the drive gear  39  disposed beneath the cover  4 , and since the thickness of the cam body secures the length L ( FIG. 1 ) of the shaft support part, the extent to which the shaft  9  extends outward from the cover  4  is minimal, and there is no uneven or axial force that tends to move the hollow shaft  9  downwardly as the operating lever  8  is turned. Thus, dust and other contaminants are not carried down underneath the cover  4  ensuring smooth rotary operation of the hollow shaft  9  and the lever  8 . 
     In the illustrated embodiment as shown in  FIGS. 8 and 9 , the fuel metering chamber  26  is communicated with the fuel supply pipe  20  via a check valve on the fuel outlet  61 , a first counterbore  73  of a fuel adjustment needle valve  34 , a gap between a passage or bore and needle  69 , and a passage  71 . 
     An antiturning body  67  for locking the fuel adjustment needle valve  34  to inhibit tampering or inadvertent adjustment of the needle valve  34  is disposed in a third counterbore  75  in the carburetor body  14 , and covered with an extended portion of a retaining plate  45  having a hole  45   a . The third counterbore  75  is larger in diameter than the hole  45   a , a second counterbore  74 , the first counterbore  73  and the passage or bore  72  which become smaller in inside diameter and are preferably coaxially aligned with the hole  45   a . A soft resin sleeve  76  is fitted in the second counterbore  74 , and a small diameter portion at one end of the resin sleeve  76  is fitted in the first counterbore  73 . A metal sleeve  77  having a tapped hole is fitted in the second counterbore  74 , and a small diameter portion at one end of the metal sleeve  77  is fitted in the resin sleeve  76 . The adjustment bolt  78  is formed with a threaded shaft portion  64 , a shaft portion  63  smaller in diameter than the inside diameter of the first counterbore  73  and a needle  69  which become narrower in order from the head  65  to the other end at the needle  69 . When the threaded shaft portion  64  is threaded in the metal sleeve  77 , the end portion of the resin sleeve  76  is placed in close contact with the first counterbore  73  by shaft portion  63 , and the needle  69  is projected into the bore  72 . 
     As shown in  FIG. 9 , after the adjustment bolt  78  has been incorporated in the carburetor, the small diameter end of the antiturning body  67  is fitted into the hole  45   a  of the retaining plate  45 , and then the retaining plate  45  is fastened to the plate  44  by a plurality of bolts  46 . The antiturning body  67  is provided with a tool engaging hole  68  and a spline hole  67   b  in engagement with a spline shaft portion  65   a  of the head  65 . At the time of shipment from the factory, the antiturning body  67  is separate from the head portion  65 . A tool such as a screw driver is engaged with a slot  66  of the adjustment bolt  78  through the tool receiving hole  68  of the antiturning body  67 , the adjustment bolt  78  is advanced or retracted to adjust the flow rate of fuel that flows from the fuel metering chamber  26  to the passage  71  via the check valve on the fuel outlet  61 , the first counterbore  73  and the passage or bore  72 . Subsequently, the spline hole  67   b  of the antiturning body  67  is advanced or pushed to fit on the spline shaft portion  65   a  of the head portion  65  (as shown in  FIG. 8 ) so that at least one and preferably, a plurality of ridges or flanges on one spline portion  67   b ,  65   a  are received in complementary grooves in the other spline portion  67   b ,  65   a . An antiturning piece key or  67   a  projecting radially outwardly from the antiturning body  67  is always engaged with an axial groove  75   a  formed on the inner peripheral surface of the third counterbore  75 . With the antiturning body fixed against rotation and coupled to the adjusmtent bolt  78 , the adjustment bolt  78  cannot be rotated. Further, since the largest outside diameter of the antiturning body  67  is larger than the diameter of the hole  45   a , the antiturning body  67  cannot be removed with the retaining plate  45  in place. Accordingly, the engine operator cannot rotate the adjustment bolt  78  of the fuel adjustment needle valve  34  arbitrarily to adjust quantity of fuel, nor can the needle valve  34  inadvertently rotate. 
     It is noted that the fuel adjustment needle valve  34  of the present invention is not limited to a diaphragm-type carburetor but can also be applied to a float bowl carburetor. Further, the fuel adjustment needle valve  34  of the invention is not limited to a rotary throttle valve system carburetor but can also be applied to a butterfly or slide-type throttle valve carburetor. 
     In the illustrated embodiment of  FIGS. 10 to 15 , a hollow shaft  9  is rotatably supported on a lid plate  4  in parallel with a rotary throttle valve, an upper end of the rotary throttle valve being covered with the lid plate  4 . A swivel  10  is rotatably supported on the free end of an operating lever  8  connected to the hollow shaft  9 , and the operating lever  8  can be turned between an idle position yieldably biased against the idle stop bolt  13  by force of a return spring, not shown, and a fully opened position placed in contact with a stop wall  96  of a cable holder  80 . The idle stop bolt  13  is threaded in a projecting wall  12  at an edge of the lid plate  4 . Although not shown, a drive gear connected to the hollow shaft  9  and a driven gear connected to the rotary throttle valve are meshed with each other, and the rotary throttle valve is rotated by the turning of the operating lever  8 . The aforementioned constitution is disclosed in Japanese Patent Application No. 2002-161710. 
     As shown in  FIGS. 10 and 13 , the cable holder  80  of the throttle valve operating mechanism is provided on the lid plate  4  opposite to the swivel  10 . The cable holder  80  is preferably formed integral with the lid plate  4 . A receiving cylinder  81  for fitting an end of an outer tube  91  covered with a metal cover  90  is formed on the right half portion of the cable holder  80 , and a slit  83  is open to the receiving cylinder  81  and an insert port  82  preferably of rectangular shape in section. The insert port  82  is reinforced by a pair of reinforcing ribs  84 , and communicated with the receiving cylinder  81  through the slit  83 . As shown in  FIG. 11 , a self-tapping hole for an adjustment bolt  86  is provided on the upper side of the slit  83  in the cable holder  80 . The adjustment bolt  86  projects toward the receiving cylinder  81  and comes in contact with the metal cover  90  or the end of the outer tube  91  through a thrust washer  87  so that an axial position of the end of the outer tube  91  with respect to the receiving cylinder  81  may be adjusted. The left end surface of the cable holder  80  constitutes a stop wall  96  of the rotary throttle valve that is engaged by the throttle valve lever  8  when the throttle valve  15  is in its wide open position. 
     When the remote control cable is mounted on the carburetor, the end of the inner wire  89  is inserted into the insert port  82 , and the end of the outer tube  91  is fitted in the receiving cylinder  81 . Since the inner wire  89  crosses the slit  83 , while the end of the outer tube  91  is pushed into the receiving cylinder  81 , the end of the outer tube  91  comes in contact with the adjustment bolt  86  through the thrust washer  87 , and the inner wire  89  projects through the stop wall  96 . Then, the end of the inner wire  89  is drawn out of the slit  83  and engaged on the split groove  10   a  of the swivel  10 . As shown in  FIG. 15 , a retainer  88  larger in diameter than that of the inner wire  89  is connected to the end of the inner wire  89 , and the retainer  88  is fitted in a cylindrical portion adjacent to the split groove  10   a  of the swivel  10 . 
     As shown in  FIG. 11 , even if the inner wire  89  is loosened, the inner wire  89  is prevented from dropping out from the swivel  10  by a flange  94  upwardly projected from the operating lever  8 . Since the end of the inner wire  89  is retained in the slit  83 , the outer tube  91  is not disengaged from the receiving cylinder  81  and the end of the inner wire  89  may be stopped at the split groove  10   a  of the swivel  10 . After the end of the inner wire  89  has been mounted in the split groove  10   a  of the swivel  10 , any “play” in the inner wire  89  is adjusted by the adjustment bolt  86  so that the rotary throttle valve may be returned to the idle position by force of a return spring, not shown. 
     The throttle valve operating mechanism of the present invention is not limited to the aforementioned embodiment, but can be also applied to, for example without limitation, other rotary throttle valve-type carburetors or slide or butterfly throttle valve-type carburetors. 
     In the carburetor as shown and described, since the rotary throttle valve is rotatably fitted in the valve chamber on the carburetor body and the annular cam is arranged against the end of the rotary throttle valve on the carburetor body, the cam can be molded, for example, of resin or the like and is easy to produce. While there is provided a pin for preventing turning of an annular cam in order to hold the annular cam on the carburetor body, it is noted that the annular cam may be pressed in and secured to the bottom of the valve chamber or may be held merely by the force of a return spring that yieldably biases the throttle valve toward its idle position. The rotary throttle valve and the annular cam are in linear or axial contact, and there is no inclination of the rotary throttle valve, providing excellent durability. 
     Since the drive gear meshed with the driven gear formed on the rotary throttle valve is rotatably supported on the support shaft secured to the carburetor body, and the hollow shaft projecting from the lid plate is operated by the operating lever, all the turning operating forces are received by the support shaft. The force for axially displacing the rotary throttle valve is not exerted on the throttle valve lever and the opening and closing of the throttle valve can be done smoothly. Since the driven gear and the drive gear are disposed in the encasing chamber which is sealed by the lid plate, no dust moves into the encasing chamber and the wear resistance and the durability of the gears and the rotary throttle valve are improved. Since the drive gear having a cam surface of the accelerating pump is connected to the end of the hollow shaft, the length of the support portion of the support shaft can be secured on the side of the carburetor body and the outwardly projecting height of the hollow shaft can be minimized. 
     The gear ratio between the driven gear and the drive gear is preferably made large and the displacement amount of the driven gear with respect to the operating amount (angle) of the drive gear is preferably made large, the amount of displacement of the swivel of the operating lever for transmitting the external operating force becomes small, and the change in angle in a pulling direction of the inner wire becomes small. 
     Since the cam surface in contact with the end of the accelerating pump is formed on the lower surface of the drive gear and the accelerating pump is disposed between the valve chamber and the support shaft, a dead space of the carburetor body can be used for the arrangement of the accelerating pump, thus contributing to the minimization of the carburetor size.