Patent Publication Number: US-7913659-B2

Title: Carburetor start system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of co-pending provisional application Ser. No. 61/074,486 filed Jun. 20, 2008, which application is fully incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates to carburetors and general purpose engines and, more particularly, to a carburetor start system which facilitates reliable starting and operation of such engines. 
     BACKGROUND 
     A variety of carburetors are used to supply fuel to general purpose two-cycle engines typically used as a source of motive power in lawn and garden machinery and the like. Such carburetors may include a butterfly throttle valve or a cylindrical throttle valve is installed crosswise in the air intake passage of the carburetor main body. The throttle valve controls the air flow rate by varying the degree of opening of the air intake passage. In a state where the throttle is released, the throttle valve is placed in a position which supplies the air and fuel necessary for idle revolution of the engine. From the idle position, the throttle valve is moved in accordance with the operation of the throttle to increase the amounts of air and fuel. 
     The most common starting system for a lawn and garden engine requires the user to complete multiple steps in order to start the engine. For example, the carburetor is first purged of old fuel and air by pushing and releasing the primer pump bulb. Second, the user must push a lever to close the choke. The third step in the process is to pull a starter rope to crank the engine until the engine starts and dies. Often the only indication that the engine tried to start is an audible pop which is difficult to identify. Step four is to put the choke lever in the half choke position and pull the starter rope again until the engine starts and runs. The final step is to put the choke lever in the run position while the engine is running and proceed to use the unit. The procedure for starting the engine will change depending on conditions such as temperature and when the unit was last used. This conventional starting process creates opportunities for mistakes and confusion. The most frequent reason for lawn and garden units being returned to the retailer is that unit will not start or is difficult to start. 
     Thus, an improved system and method that facilitates reliable starting and operation of such engines is desirable. 
     SUMMARY 
     The various embodiments and examples provided herein are generally directed to carburetors comprising a start system that facilitates reliable starting and operation of general purpose engines. In preferred embodiments described herein, the conventional multi-step starting process is advantageously reduced by combining into a single step the steps of purging of the carburetor of old fuel and activating of the starting system in which a throttle and/or a choke valve are opened to a starting position. The present system provides the user with a starting procedure that is simplistic and less prone to operator error. The system functions the same regardless of conditions. 
     In operation, the start system fills the carburetor with fuel and places a throttle and/or choke valve in a predetermined start position in a single step as a primer pump bulb is pushed. More particularly, the start system translates the axial force used to push the primer pump bulb into rotational movement of the throttle and/or choke valve. 
     Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The details of the invention, both as to its structure and operation, may be gleaned in part by study of the accompanying figures, in which like reference numerals refer to like parts. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, all illustrations are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than literally or precisely. 
         FIGS. 1 through 6  illustrate a butterfly or disc throttle valve carburetor embodiment with a start system that facilitates reliable starting;  FIGS. 1 ,  3  and  5  are plan views of the carburetor in a “starting” orientation, and  FIGS. 2 ,  4  and  6  are plan views of the carburetor in a “running” orientation. 
         FIG. 7  is an exploded isometric view of a preferred embodiment of a carburetor start system incorporated with a start pump circuit. 
         FIGS. 8A and 8B  are partial cross-sectional views taken along line  8 - 8  in  FIG. 7 ;  FIG. 8A  depicts the carburetor in a “starting” orientation, and  FIG. 8B  depicts the carburetor in a “running” orientation. 
         FIG. 9  is a top view of an auxiliary start pump body of the carburetor start pump circuit shown in  FIG. 7 . 
         FIG. 10  is a partial cross-sectional view taken along line  10 - 10  in  FIG. 9 . 
         FIG. 11  is a cross-sectional view taken along line  11 - 11  in  FIG. 9 . 
         FIGS. 12 through 17  illustrate a butterfly or disc throttle valve carburetor embodiment with a choke and a start system that facilitates reliable starting;  FIGS. 12 ,  14  and  16  are plan views of the carburetor in a “running” orientation, and  FIG. 13 ,  15  and  17  are plan views of the carburetor in a “starting” orientation. 
         FIGS. 18 and 19  illustrate a rotary throttle valve carburetor embodiment with a start system that facilitates reliable starting;  FIG. 18  is a plan view of the carburetor in a “starting” orientation, and  FIG. 19  is a plan view of the carburetor in a “running” orientation. 
         FIG. 20  is a longitudinal sectional view of the rotary throttle valve carburetor embodiment shown in  FIGS. 18 and 19  taken along line  20 - 20  in  FIG. 16 . 
         FIG. 21  is a partial plan view of the embodiment shown in  FIG. 20 . 
         FIGS. 22 and 23  illustrate the placement of the cam part in the operative position;  FIG. 22  is a partial longitudinal sectional view, and  FIG. 23  is a partial plan view. 
         FIGS. 24 and 25  illustrate alternative embodiments depicting a cam ramp on the under-side of the throttle lever. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Each of the additional features and teachings disclosed below can be utilized separately or in conjunction with other features and teachings to provide carburetors with a start system that facilitates reliable starting and operation of general purpose engines. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in combination, will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Therefore, combinations of features and steps disclosed in the following detail description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the present teachings. 
     Moreover, the various features of the representative examples and the dependent claims may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. In addition, it is expressly noted that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure, as well as for the purpose of restricting the claimed subject matter independent of the compositions of the features in the embodiments and/or the claims. It is also expressly noted that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure, as well as for the purpose of restricting the claimed subject matter. 
     Embodiments of the present invention are described below with reference to the drawings and are directed to a start system that fills the carburetor with fuel and places a throttle and/or choke valve in a predetermined start position in a single step. More particularly, the start system translates the axial force used to depress a primer pump bulb into rotational movement of the throttle and/or choke valve. 
     Referring to  FIGS. 1 through 6 , an embodiment of a carburetor start system is illustrated. As depicted, a carburetor  10  includes a main body  1  with an air intake passage  2  passing longitudinally through the carburetor main body  1 . A conventional butterfly throttle valve  3  is provided so that both ends of a valve shaft  4  protrude from the body  1 . The throttle valve  3  comprises a round valve plate  5  attached to the valve shaft  4  rotatably retained in the body  1  and crossing the intake channel  2 . Opening and closing of the throttle valve  3  is conducted by a well-known conventional method, for example, by tension rotating a throttle valve lever arm  6  secured to one end of the valve shaft  4  by an acceleration operation, or by an elastic force of a return spring  7  consisting of a helical coil spring installed at the same end of the shaft  4  and actuated by the throttle valve lever arm  6 . 
     A carburetor start system  50  comprises a primer pump bulb  52  operably coupled to a valve-opening mechanism  20 . The primer pump bulb  52 , which is in fluid communication with a fuel metering chamber and the air intake passage  2 , includes a base body  56  coupled to a first end of a starter shaft  22  slidably retained in the carburetor body  1 . The starter shaft  22  traverses the air intake passage  2  and protrudes at both ends from opposite sides of the carburetor body  1 . A start arm  24  having a cam surface  26  is coupled to the starter shaft  22  on an opposite end of the shaft  22 . The cam surface  26  and a follower surface  9  on a follower arm  8 , which is secured to the throttle valve shaft  4  at an end opposite the throttle valve lever  6 , translate the axially movement of the starter shaft  22  into rotation movement of the throttle shaft  4 . 
     In operation, as the primer pump bulb  52  is initially pressed to purge the carburetor  10  of old fuel, the shaft  22  and start arm  24  move axially and a follower surface  9  on the follower arm  8  coupled to the throttle shaft  4  follows the cam surface  26  on the start arm  24  causing rotation of the throttle shaft  4  and valve plate  5  to fast idle position that will make starting easier. Locking surfaces  9   a  and  28  on the follower arm  8  and the start arm  24 , respectively, engage to lock the start arm  24  and follower arm  8  in place. The primer pump bulb  52  can be pushed or depressed several more times, preferably about five (5) to seven (7) times, to fill the carburetor  10  with fuel. With the start arm  24  and follower arm  8  fixedly engaged, and the carburetor  10  primed with fuel, the operator can pull a pull or crank rope or cord to start an engine to which the carburetor is coupled. 
     As the throttle lever arm  6  rotates in a run mode or during operation of the engine, the start arm  24  is released from engagement with the follower arm  8  and the shaft is caused to translate to its original or running position by an axial return spring  58 . The axial return spring  58  is coiled about the starter shaft  22  between the base body  56  and the carburetor body  1  and biases the primer pump bulb  52 , shaft  22  and start arm  24  towards a running position wherein the start arm  24  is disengaged from and does not interfere with the operation of the follower arm  8 . 
     Turning to  FIGS. 7-11 , the carburetor start system is shown incorporated with a start pump circuit. Referring to  FIG. 7 , a relatively standard carburetor body  210  includes a main pulse passageway  216  bored into the carburetor body  210  from its face  217 . The main pulse passageway  216  opens into a pulse chamber  215  of a main fuel pump  211  bored into the carburetor body  210  from a top surface  213 . A starting pulse passageway  220  is also bored into the carburetor body  210  from the face  217 . A channel  218 , preferably about two millimeters wide, is cut into the face  217  of the carburetor body  210 . The channel  218  runs from the main pulse passageway  216  to the starting pulse passageway  220  to carry the crankcase pulse to the starting pulse passageway  220 . The channel  218  is interconnected to the crank case of an engine at a point adjacent to the main pulse passageway  216 . The remainder of the channel  218  is covered by a carburetor mounting gasket (not shown) which interposes the carburetor body  210  and the engine (not shown) when the carburetor is mounted on the engine. 
     Three passageways are bored into the carburetor body  210  from the top surface  213 . The first is a pulse passageway  224  which opens into the starting pulse passageway  220 . The second is a fuel intake passageway  222  which opens into the metering chamber (not shown) of the carburetor body  210 . The third is a fuel discharge passageway  226  which opens into a throttle bore  214  of the air intake of the carburetor body  210 . 
     A pair of holes  221  and  223  are tapped into the top surface  213  of the carburetor body  210  and used to mount a main fuel pump  211  and an auxiliary or start fuel pump  212 . The main fuel pump  211 , which operates in a manner known in the art, includes a flat fuel pump diaphragm  230  mounted on the top surface  213  of the carburetor body  210 . The diaphragm  230  interposes the carburetor body  210  and a fuel pump gasket  240 . The fuel pump diaphragm  230  includes a pair of holes  231  and  233  that are aligned with holes  221  and  223  in the carburetor body  210  to mount the diaphragm  230  on the carburetor body  210 . In addition, the fuel pump diaphragm  230  includes a fuel intake hole  232 , a pulse hole  234 , and a fuel discharge hole  236 . The fuel intake hole  232 , the pulse hole  234 , and the fuel discharge hole  236 , respectively, are aligned with the fuel intake passageway  222 , the pulse passageway  224 , and the fuel discharge passageway  226 , respectively, in the carburetor body  210  when the fuel pump diaphragm  230  is mounted on the top surface  213  of the carburetor body  210 . 
     The fuel pump gasket  240 , which mounts on the carburetor body  210  on top of the fuel pump diaphragm  230 , also includes a pair of holes  241  and  243  that are aligned with holes  221  and  223  in the carburetor body  210  to mount the gasket  240 . The fuel pump gasket  240  also includes a fuel intake hole  242 , a pulse hole  244 , and a fuel discharge hole  246 , respectively, that are aligned with the fuel intake passageway  222 , the pulse passageway  224 , and the fuel discharge passageway  226 , respectively, in the carburetor body  210  when the fuel pump gasket  240  is mounted on the carburetor body  210 . 
     The auxiliary fuel pump  212  includes a pump body  250  mounted on top of the main fuel pump  211 , a start pump gasket  270  mounted on top of the start pump body  250 , a start pump diaphragm  280  mounted on top of the start pump gasket  270  and a start pump cover  290  mounted on top of the start pump diaphragm  280 . Holes  251  and  253  in the start pump body  252 , holes  271  and  273  in the start pump gasket  270 , holes  281  and  283  in the start pump diaphragm  280 , and holes  291  and  293  in the start pump cover  290  are all aligned with the holes  221  and  223  in the carburetor body  210  to mount these components on the carburetor body  210 . 
     The auxiliary pump body  250  as shown in  FIGS. 7 ,  9 ,  104  and  11  includes a fuel intake passageway  252  bored into the fuel pump body  250  from its bottom surface  258 . The fuel intake passageway  252  opens into an intake pathway  255  bored into the auxiliary pump body  250  from its side  259 . A plug  257  seals one end of the intake pathway  255  adjacent to the side  259  of the pump body  250 . The intake pathway  255  directs the fuel from the metering chamber to an inlet check valve  262  seated in an inlet valve chamber  261 . The inlet check valve  262  is a simple viton disk that allows fuel to flow into a pumping chamber  260  bored into the auxiliary pump body  250  from its top surface  267 , but prevents back flow. The inlet valve chamber  261  is bored into the auxiliary pump body  250  from the pump chamber  260 . A calibrated inlet jet  263  may be positioned at the entrance of the inlet check valve  262  to meter the flow of fuel into the pumping chamber  260 . 
     The auxiliary pump body  250  also includes a pulse passageway  254  bored through the auxiliary pump body  250  and a fuel discharge passageway  256  bored into the auxiliary pump body  250  from its bottom surface  258 . The pulse passageway  254  is aligned with the pulse passageway  224  in the carburetor body  210  and the fuel discharge passageway  256  is aligned with the fuel discharge passageway  226  in the carburetor body  210 . The fuel discharge passageway  256  opens to a discharge check valve chamber  265  bored into the auxiliary pump body  250  from the pumping chamber  260 . A discharge check valve  266  is mounted in the valve chamber  265 . The discharge check valve  266  is held close against its seat by a spring  268  positioned on the discharge side of the check valve  266 . The spring force prevents fuel from being drawn out of the system by the carburetor manifold vacuum when the start pump  212  is shut off, i.e., when the throttle shaft  225  is rotated out of the start position and passageway  228  is no longer aligned with passageways  220  and  224 . A calibrated jet  264  may be positioned on the inlet side of the discharge check valve  266 . The calibrated jets  263  and  264  restrict the fuel flow into the engine to prevent an over-rich condition at startup. 
     The auxiliary pump gasket  270  maintains a seal between the auxiliary pump body  250  and the auxiliary pump diaphragm  280 . The gasket  270  includes a pulse hole  274  aligned with the pulse passageway  224  in the carburetor body  210  and a hole  275  aligned with the pumping chamber  260  in the auxiliary pump body  250  to allow the auxiliary pump diaphragm  280  to communicate with the pumping chamber  260 . 
     The auxiliary pump diaphragm  280  transfers the force of the crank case pulse to the fuel in the pumping chamber  260  of the auxiliary pump body  250 . The flat auxiliary pump diaphragm  280  includes a pulse hole  284  aligned with the pulse passageway  224  in the carburetor body  210 . 
     The pump cover  290 , which seals the stack of gaskets  240  and  270 , diaphragms  230  and  280 , and the auxiliary pump body  252 , accepts the crank case pulse P and directs it to the auxiliary pump diaphragm  280 . 
     Referring back to FIGS.  7  and  8 A- 8 B, a conventional butterfly throttle valve  203  is provided so that both ends of a valve shaft  225  protrude from the carburetor body  210 . The throttle valve  203  comprises a round valve plate  205  attached to the valve shaft  225  rotatably retained in the body  210  and crossing the intake channel  214 . Opening and closing of the throttle valve  2033  is conducted by a well-known conventional method, for example, by tension rotating a throttle valve lever arm  227  secured to one end of the valve shaft  225  by an acceleration operation, or by an elastic force of a return spring (see  FIGS. 3 and 4 ) consisting of a helical coil spring installed at the same end of the shaft  225  and actuated by the throttle valve lever arm  227 . 
     As depicted, a carburetor start system  201  is provided. The start system  201  comprises a primer pump bulb  206  operably coupled to a valve-opening mechanism  207 . The primer pump bulb  206  is coupled to a first end of a starter shaft  204 , which is slidably retained in the carburetor body  210 . The starter shaft  204  traverses the air intake passage  214  and protrudes at both ends from opposite sides of the carburetor body  210 . A start arm  202  having a cam surface is coupled to the starter shaft  204  on an opposite end of the shaft  204  from the primer bulb  206 . The cam surface on the start arm  202  and a follower surface on a follower arm  229 , which is secured to the throttle valve shaft  225  at an end opposite the throttle valve lever  227 , translate the axially movement of the starter shaft  204  into rotation movement of the throttle shaft  225 . As the primer pump bulb  206  is pressed to purge the carburetor of old fuel, the shaft  204  and start arm  202  move axially and the follower surface on the follower arm  229  coupled to the throttle shaft  225  follows the cam surface on the start arm  202  causing rotation of the throttle shaft  225  and valve plate  205  to a fast idle position that will make starting easier. Locking surfaces and on the follower arm  229  and the start arm  202 , respectively, engage to lock the start arm  202  and follower arm  229  in place. The primer pump bulb  206  can be pushed or depressed several more times, preferably about five (5) to seven (7) times, to fill the carburetor with fuel. With the start arm  202  and follower arm  229  fixedly engaged, and the carburetor primed with fuel, the operator can pull a pull or crank rope or cord to start an engine to which the carburetor is coupled. 
     As the throttle lever arm  227  rotates in a run mode or during operation of the engine, the start arm  202  is released from engagement with the follower arm  229  and the shaft is caused to translate to its original or running position by an axial return spring (see  FIGS. 1-4 ). The axial return spring is coiled about the starter shaft  204  between the primer bulb  206  and the carburetor body  210  and biases the primer pump bulb  206 , starter shaft  204  and start arm  202  towards a running position wherein the start arm  202  is disengaged from and does not interfere with the rotation of the follower arm  229 . 
     As shown in detail in  FIGS. 8A and 8B , the pulse passageway  224  is shown to be operably interconnected to the starting pulse passageway  220  via a passageway or channel  228  cut into the starter shaft  204 . Thus, passageways  220  and  224  are only in communication with one another when the starter shaft  204  and start arm  202  and, thus, the throttle valve plate  205  and throttle lever  227 , are positioned in a start position which results in passageway  228  being aligned with passageway  224 , as show in  FIG. 8A . 
     In operation, the start pump  250  is activated by turning on the crank case pulse supplied to it. As noted, the crank case pulse P is controlled with the starter shaft  204  as shown in  FIGS. 8A and 8B , or similarly by a choke shaft or the like. In the preferred embodiment, the passageway  228  in the starter shaft  204  aligns when in a start position with passageway  224  in the carburetor body  210  and the pulse P is allowed to enter the start pump  212  when the primer bulb  206  is depressed causing the starter shaft  204  and start arm  202  to move axially and lock the start arm  202  and follower arm  229  in a start position. This control configuration ensures that the start pump  212  only feeds fuel to the engine during start-up. 
     The pulse P travels up through the stack of the main fuel pump diaphragm  230  and the main fuel pump gasket  240 , and then through the auxiliary pump body  252 , diaphragm  280 , and gasket  270  and on into the start pump cover  290 . The pulse P moves the diaphragm  280  up and down which creates a corresponding vacuum and pressure in the pumping chamber  260  of the auxiliary pump body  250 . The vacuum pulse opens the inlet check valve  262  and draws fuel I from the metering chamber (not shown) of the carburetor body  210 . By drawing fuel from the metering chamber, the carburetor start pump circuit advantageously acts as an air purge or primer. 
     The fuel I passes through the carburetor body  210  through the main fuel pump diaphragm  230  and gasket  240 , into the start pump body  250  and on into the pumping chamber  260  through the inlet check valve  262  and, optionally, through the calibrated metering jet  263 . When the auxiliary pump diaphragm  280  is pushed down into the auxiliary pump body  250  by the crank case pulse P, the inlet check valve  262  is forced closed and the force of the crank case pulse P is transferred to the fuel forcing the fuel through the discharge check valve  266  and, optionally, first through the calibrated metering jet  264 . The fuel must pass through the starting jet  264  and press open the spring  268  loaded check valve  266  to leave the pumping chamber  260 . The spring  268  exerts a sufficient force on the check valve  266  to prevent it from being opened by a manifold vacuum and thus ensuring that fuel is not drawn through the carburetor start pump circuit unless the start pump  212  is receiving a pulse P. 
     The fuel D then exits the auxiliary pump body  250  through the discharge fuel passageway  256  and passes back through the main pump gasket  240  and diaphragm  230 , and on through the fuel discharge passageway  226  into the throttle bore  214  in the carburetor body  210 . When the engine is warmed up, the operator shuts off the start pump circuit and the engine begins normal operation. 
     As mentioned above, often times the operator may neglect to shut off the start pump circuit when the engine is warmed up or accidentally engage the start pump when the engine is already operating and warmed up. This may result in the engine stalling or “conking out” from too much fuel being discharged into the throttling bore  214 . One approach to prevent the engine from stalling is to place a calibrated restriction or jet  285  anywhere along the path that the start pulse P travels, and preferably somewhere between the carburetor body  210  and the start pump cover  290  of the auxiliary fuel pump  212 . The jet  285  is placed in the pulse hole  284  of the start pump diaphragm  280 . 
     The jet  285  is positioned and calibrated such that the jet  285  tends to substantially choke off high frequency pulses P transmitted from the engine, thus substantially choking off the power to move the start pump diaphragm  280  at the high frequencies. In other words, when the engine starts to warm up, the jet  825  tends to substantially reduce the amount of fuel D that the auxiliary fuel pump  212  discharges into the throttling bore  214 . 
     When the engine is being cranked, a low frequency pulse P, e.g., about 18 hz or about 800 rpm, is transmitted from the engine. At the lower frequency, a substantial portion of the pulse P will pass through the jet  285  sufficient to operate the start pump diaphragm  280 . When the engine starts to warm up, it starts to supply a higher frequency pulse P, e.g., about 80 hz or about 5000 rpm. At this point, the engine will no longer need mixture enrichment. The jet  285  tends to choke off a substantial amount of the pulse P transmission to the start pump circuit sufficient to substantially decrease the operation of the start pump diaphragm  280 . Thus, the start pump circuit will advantageously cease operation or at least substantially limit the amount of fuel D discharged into the throttling bore  214 , preventing the engine from conking out or stalling. 
     Referring to  FIGS. 12 through 17 , another embodiment of a carburetor start system is illustrated. As depicted, as with the previous embodiment described above, the carburetor  11  includes a main body  1  with an air intake passage  2  passing longitudinally through the carburetor main body  1 . A conventional butterfly throttle valve  3  is provided so that both ends of a valve shaft  4  protrude from the body  1 . The throttle valve  3  comprises a round valve plate  5  attached to the valve shaft  4  rotatably retained in the body  1  and crossing the intake channel  2 . Opening and closing of the throttle valve  3  is conducted, for example, by tension rotating a throttle valve lever arm  6  secured to one end of the valve shaft  4  by an acceleration operation, or by an elastic force of a return spring  7  consisting of a helical coil spring installed at same end of the shaft  4  and actuated by the throttle valve lever arm  6 . 
     In addition to the above, a choke valve  12  is provided so that both ends of a valve shaft  14  protrude from the body  1 . The choke valve  12  comprises a valve plate  13  attached to the valve shaft  14  rotatably retained in the body  1  and crossing the intake channel  2 . A choke lever  15  is attached to a first end the valve shaft  14  on the throttle lever arm  6  side of the carburetor body  1 . The choke lever  15  includes a lever arm  16  with a locking surface  16   a  that when rotated a predetermined amount abuts an end  17   a  of a locking arm  17  coupled to an end of the throttle valve shaft  4  opposite the throttle lever arm  6  to fix choke valve  12  in a starting position. 
     A carburetor start system  50  comprises a primer pump bulb  52  operably coupled to a choke valve-opening mechanism  21  and, optionally, also to a throttle valve-opening mechanism  20 . The primer pump bulb  52  includes a base body  56  coupled to a cylindrical sleeve  54  slidably received over an end of the choke valve shaft  14  opposite the choke lever arm  15 . The primer pump bulb  52  further comprises a retainer arm  57  coupled to the base body  56  and slidably received over a guide arm  18  extending from the carburetor body  1  to maintain the orientation of the primer bulb  52  relative to the choke valve shaft  14 . The retainer arm  57  and guide arm  18  preferably include a key or protrusion  57   a  extending from the retainer arm  57  and received in a groove or keyway  18   a  formed in the guide arm  18 . 
     The choke valve opening mechanism preferably includes a cam surface  19  cut into the valve shaft  14  and a follower  55  extending from the base  56  into the interior of the sleeve  54  that engages the cam surface  19  to translate the axially movement of the sleeve  54  into rotational movement of the choke shaft  14 . As the primer pump bulb  52  is pressed to purge old fuel from the carburetor, the sleeve  54  moves axially toward the choke lever arm  15  with the follower  55  engaging the cam surface  19  causing the choke valve shaft  14  and valve plate  13  to rotate to a closed or starting position. By rotating the shaft  14  and valve plate  13  to a starting position, the locking surface  16   a  of the choke lever arm  16  abuts the end  17   a  of the locking arm  17  fixing the choke valve  12  in the starting position. The primer pump bulb  52  can be pushed or depressed several more times, preferably about five (5) to seven (7) times, to fill the carburetor  11  with fuel. With the choke lever arm  16  and locking arm  17  fixedly engaged, and the carburetor  11  primed with fuel, the operator can pull a pull or crank rope or cord to start an engine to which the carburetor is coupled. 
     As the throttle shaft  4  rotates in a run mode or during operation of the engine, the choke lever arm  16  is released from engagement with the locking arm  17  and the choke valve shaft  14  is caused to rotate by the bias of an axial return spring  58  applied to the sleeve  54  to its original or running position with the choke valve plate  13  in an open position. The axial return spring  58  is coiled about the sleeve  54  between the base body  56  and the carburetor body  1  and biases the primer pump bulb  52  and sleeve  54  in a direction away from the carburetor body  1  and towards a running position wherein the choke lever arm  16  is unengaged from and does not interfere with the movement of the locking arm  17  on the throttle shaft  4 . 
     As noted above, the carburetor start system  50  optionally includes a throttle valve opening mechanism  21  that comprises a start arm  24  having a cam surface  26  extending from the base body  56  of the primer pump bulb  52 . As the primer pump bulb  52  is pressed purge old fuel from the carburetor  11 , the start arm  24  moves axially and a follower surface  8  on the throttle lever arm  6  coupled to the throttle shaft  4  follows the cam surface  26  on the start arm  24  causing rotation of the throttle shaft  4  and valve plate  5  to a fast idle position—i.e., preferably rotated about  20  to  30  degrees. Locking surfaces  9  and  28  on the throttle lever arm  6  and the start arm  24  engage to lock the start arm  24  and throttle lever arm  6  in place. The primer pump bulb  52  can be pushed or depressed several more times, preferably about five (5) to seven (7) times, to fill the carburetor  11  with fuel. With the start arm  24  and throttle level arm  6  fixedly engaged, and the carburetor  11  primed with fuel, the operator can pull a pull or crank rope or cord to start an engine to which the carburetor is coupled. As the throttle lever arm  6  rotates in a run mode or during operation of the engine, the start arm  24  is released from engagement with the throttle lever arm  6 . 
     Referring  FIGS. 18 through 25 , another embodiment of a carburetor start system is illustrated. As depicted, a carburetor  100  includes a main body  101  with an air intake passage  102  passing longitudinally through the carburetor main body  101 , and a valve hole  103  which is perpendicular to the air intake passage  102 , and which is closed at one end. A cylindrical throttle valve  104  is inserted into the valve hole  103  so that said throttle valve  104  can rotate, and so that said throttle valve  104  can move in the central axial direction. 
     The throttle valve  104  has a throttle orifice  105  which is perpendicular to the central axial line of the throttle valve  104  and which has approximately the same diameter as the air intake passage  102 . The throttle valve  104  also has a nozzle insertion orifice  106 , a metering needle  107  and a valve shaft  108  which are installed on the central axial line of the throttle valve  104 . The nozzle insertion orifice  106  is formed in the end portion located at the closed end of the valve hole  103 . The valve shaft  108  is an integral part of the throttle valve  104 . The valve shaft  108  extends from the end portion of the throttle valve  104  located at the open end of the valve hole  103 , and passes through the cover body  110  of the valve hole  103  so that said valve shaft  108  protrudes to the outside of the carburetor main body  101 . The metering needle  107  is fastened in the throttle valve  104  in such a manner that the distance by which said metering needle  107  protrudes into the throttle orifice  105  can be adjusted by screwing a screw head part  107   a  at the base end of the metering needle  107  into a screw hole  109 . 
     A lever  111 , which is turned by the operation of the throttle by an operator, is fastened to the shaft end of the valve shaft  108 . A push spring  112  consisting of a compression coil spring is mounted between the cover body  110  and the throttle valve  104  so that said push spring  112  surrounds the valve shaft  108 . A groove cam  113  is formed in the outer circumferential surface of the throttle valve  104  so that the groove cam  113  extends around roughly one-fourth of the circumference of the throttle valve  104 . A supporting pin  114  which is screwed into the carburetor main body  101  is inserted and engaged in the groove cam  113 . 
     Alternatively, as shown in  FIGS. 24 and 25 , a cam ramp  160  is formed on an underside of the lever  111 . A follower pin  161  extends from cover body  110  and engages the ramp cam  160 . 
     When the lever  111  is turned by the operation of the accelerator, the throttle valve  104  rotates as a unit with the lever  111 , thus causing the degree of overlap between the throttle orifice  105  and the air intake passage  102  to vary so that the intake air flow rate of the engine is controlled. At the same time, the throttle valve  104  moves along the central axial line in accordance with the groove cam  113  or cam ramp  160 , thus causing the depth of insertion of the metering needle  107  into the fuel nozzle  115  to vary so that the fuel flow rate is controlled. This operation is the same as that of a conventional rotary throttle valve type carburetor. 
     A constant-fuel chamber  116  which is the same as that of a well-known diaphragm type carburetor is formed in the opposite end surface of the carburetor main body  101  from the cover body  110 . The fuel chamber  116  is separated from the atmosphere by a diaphragm. The fuel in the constant-fuel chamber  116  passes through a fuel passage  117 , and is blown into the throttle orifice  105  from the fuel nozzle  115  and thus supplied to the engine. 
     A fuel pump  118  is installed on the outside of the constant-fuel chamber  116 . This fuel pump  118  is a well-known pump in which the diaphragm is operated by the pulse pressure generated in the crankcase of the engine, so that fuel in the fuel tank is supplied to the constant-fuel chamber  116 . 
     A carburetor start system  150  comprises valve-opening mechanism  121  installed on the cover body  110  and operably coupled to a primer pump bulb  152 . The valve-opening mechanism  121  is equipped with a substantially square cam part  122  which performs a linear reciprocating movement along the outside surface of the cover body  110 , and a return spring  123  which places the cam part  122  in an inoperative position. The cam part  122  is passed through a gate-formed guide part  124  which protrudes from the outside surface of the cover body  110 . 
     The base end surface of the cam part  122  is formed as a flat pushing surface  125 . A base  156  of the primer pump bulb  152  is coupled to the cam part  122  and abuts the pushing surface  125 . A first cam surface  126  formed on the tip end portion  122   a  of the cam part  122  contacts the side surface  111   a  of the lever  111  and pushes the lever  111  in the direction that increases the air flow rate. A second cam surface  127 , also formed on the tip end portion  122   a  of the cam part  122 , contacts the tip edge  111   b  of the lever  111  and pushes the lever  111  so that the lever  111  is caused to move in the axial direction that increases the fuel flow rate. A holding surface  128  which overlaps with the tip end portion  111   c  of the inside surface of the lever  111  is also formed on the tip end portion  122   a  of the cam part  122 . An engaging groove  129  is formed in the holding surface  128 . The portion of the tip edge  111   b  of the lever  111  which contacts the second cam surface  127  forms an engaging pawl or detent  111   d  that is inserted into the engaging groove or detent pocket groove  129 . 
     A groove hole  130  in which a portion of the return spring  123  is mounted, and a projection  131  which is used for stroke regulation, are formed in the base end portion of the cam part  122 . A cut-out groove  132  in which a portion of the return spring  123  is mounted is formed in one edge of the cover body  110 . The above-mentioned guide part  124  is disposed on a receiving edge  133  which protrudes outwardly from the carburetor main body  101 . A regulating groove  134  used for stroke regulation is formed in the inside surface of the guide part  124 . 
     The aforementioned cam part  122  is passed through the guide part  124  so that the tip end portion  122   a  of the cam part  122  overlaps with the cover body  110 , and so that the base end portion of the cam part  122  overlaps with the receiving edge  133 . The stroke of the cam part  122  in the longitudinal direction is regulated by a projection  131  which is inserted into the regulating groove  134 . The cam part  122  is held in the inoperative position (in which the cam part  122  is withdrawn in a direction toward its base end) by the above-mentioned return spring  123  (consisting of a compression coil spring) which is mounted in the cut-out groove  132 . 
     While in the idle position, the side surface  111   a  and tip edge  111   b  of the lever  111 , respectively, contact the first cam surface  126  and second cam surface  127 , respectively, or are slightly separated from said cam surfaces  126  and  127 , respectively. 
     In order to start the engine, the operator&#39;s fingertips are pressed against the primer pump bulb  152  causing old fuel to be purged and new fuel to enter the carburetor  100  through a primer tube  154 . Since the primer pump bulb base abuts the pushing surface  125 , as the operator&#39;s fingertips are pressed against the primer pump bulb  152 , the cam part  122  is caused to advance wherein the first cam surface  126  pushes the side surface  111   a  so that the lever  111  is caused to turn in the direction that increases the air flow rate. At the same time, the second cam surface  127  pushes the tip edge  111   b  so that the lever is caused to move in the direction that increases the fuel flow rate. However, the lever  111  stops when the engaging pawl  111   d  engages in the engaging groove  129 . 
     The second cam surface  127  is formed with an angle of inclination which is equal to or greater than that of the groove cam  113  or cam ramp  160 . As a result of the aforementioned movement of the lever  111 , the degree of overlap between the air intake passage  102  and the throttle orifice  105  of the throttle valve  104  is slightly increased, and the depth of insertion of the metering needle  107  into the fuel nozzle  115  is slightly reduced, so that the amounts of air and fuel necessary for starting are supplied to the engine. The angle of inclination of the second cam surface  127  is preferably set at a larger value than the angle of inclination of the groove cam  113  or cam ramp  160 , so that the increase in the fuel flow rate is greater than the increase in the air flow rate. 
     The tip end portion  111   c  of the inside surface of the lever  111  is pressed against the holding surface  128  by the spring force of the push spring  112 , so that even if the fingers are removed, the cam part  122  is fixed in the operative position by the frictional force generated between the above-mentioned parts, and is not returned by the spring force of the return spring  123 . 
     The lever  111  is mechanically coupled with the cam part  122  by an anchoring means  136  comprising of the engaging pawl  111   d  and engaging groove  129 . Accordingly, the lever  111  is stably fixed in the operative position so that starting of an engine of a lawn and garden machine can be performed by pulling a start or crank cord or rope. 
     When warm-up of the engine is completed, and a transition to normal operation is to be made, the lever  111  is caused to turn in the direction of increase of fuel and air by ordinary operation of the throttle. As a result, the engaging pawl  111   d  is released from the engaging groove  129  at more or less the same time. Furthermore, the tip end portion  111   c  of the inside surface is separated from the holding surface  128  so that the cam part  122  is returned to the inoperative position by the spring force of the return spring  123 . Afterward, the lever  111  can be turned from the idle position to the full-open position by operation of the accelerator, without being constrained by the cam part  122 . 
     The cam part  122  returns to the inoperative position (where the return spring  123  recovers its extended length), and remains in this position. In the working configuration shown in the figures, the stroke regulating means  137 , comprising the projection  131  and regulating groove  134 , prevents the cam part  122  from advancing to an excessive degree wherein the lever  111  is turned more than is necessary. In addition, this stroke regulating means  137  eliminates any concern that the cam part will be withdrawn beyond the inoperative position wherein the cam part  122  would fall out of the cover body  110  and carburetor main body  101 . Moreover, in cases where no anchoring means  136  is provided, this stroke regulating means  137  enables the cam part  122  to move to a fixed operative position so that stable starting can be accomplished. 
     Instead of inserting the cam part  122  into a gate-formed guide part  124 , it would also be possible to cause movement between the inoperative position and the operative position using a dovetail groove or other well known sliding guide means. Furthermore, instead of using a compression coil spring, it would also be possible to use a hollow or solid block consisting of a highly elastic material, e.g., rubber, as the return spring  123 . 
     As was described above, the present invention is devised so that a lever which transmits the operation of the accelerator to the throttle valve is turned slightly from the idle position by a cam part which causes the throttle valve to be held in a state that increases the amounts of air and fuel supplied to the engine. Accordingly, starting of the engine at low temperatures can be reliably accomplished by means of an extremely simple operation. Furthermore, the transition to normal operation by means of the accelerator can be smoothly accomplished. 
     Moreover, in cases where an anchoring means for the lever and cam part and a stroke regulating means for the cam part are provided, starting can be accomplished even more reliably. 
     While the above description contains many specifics, these should not be construed as limitations on the scope of the invention, but rather as examples of particular embodiments thereof. Many other variations are possible. Accordingly, the scope of the present invention should be determined not by the embodiments described herein, but by the appended claims and their legal equivalents.