Abstract:
A diaphragm-type carburetor has a body defining at least in part an air intake passage, a fuel metering chamber that receives fuel for delivery into the air intake passage, a fuel supply passage communicating a supply of fuel with the fuel metering chamber, and a bypass passage communicating the fuel supply passage with the air intake passage to facilitate removal of air and fuel vapor from the carburetor. The bypass passage is routed around, and does not communicate directly with the fuel metering chamber. Fuel vapor or air in the carburetor is quickly led from the bypass passage to the air intake passage to prevent a large quantity or volume of fuel vapor or air from staying in or flowing to the fuel metering chamber and thereby adversely affecting the operation of the carburetor.

Description:
REFERENCE TO RELATED APPLICATION 
     Applicant claims priority of Japanese patent application, Ser. No. 2001-367375, filed Nov. 30, 2001. 
     FIELD OF THE INVENTION 
     The present invention relates to a carburetor, and more particularly to a diaphragm-type carburetor that provides a fuel and air mixture to an engine. 
     BACKGROUND OF THE INVENTION 
     Diaphragm-type carburetors are commonly used to supply a fuel and air mixture to an engine in accordance with engine demand. Such carburetors use a diaphragm to control at least in part the flow of liquid fuel in the carburetor in response to a signal applied to the diaphragm. 
     The presence of air and fuel vapor in the carburetor can inhibit liquid fuel flow in the carburetor. This can make it difficult to start an engine, and even after the engine is started, can cause unstable engine operation if the fuel vapor is not purged quickly. Air and fuel vapor can become present in the carburetor, for example, when an engine is not operated for a long period of time, or a hot engine is left out in hot ambient conditions. 
     SUMMARY OF THE INVENTION 
     A diaphragm-type carburetor has a body defining at least in part an air intake passage, a fuel metering chamber that receives fuel for delivery into the air intake passage, a fuel supply passage communicating a supply of fuel with the fuel metering chamber, and a bypass passage communicating the fuel supply passage with the air intake passage to facilitate removal of air and fuel vapor from the carburetor. The bypass passage is routed around, and does not communicate directly with the fuel metering chamber. Fuel vapor or air in the carburetor is quickly led from the bypass passage to the air intake passage to prevent a large quantity or volume of fuel vapor or air from staying in or flowing to the fuel metering chamber and thereby adversely affecting the operation of the carburetor. The present invention may be used in carburetors of substantially any type including without limitation those having piston, rotary, butterfly, barrel or slide type throttle valves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front sectional view of a diaphragm type carburetor according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a carburetor body  60  is formed with an air intake passage  36  extending therethrough, a cylindrical valve chamber  20  perpendicular to the air intake passage  36 , and a piston type throttle valve  10  is slidably received in the valve chamber  20 . The valve chamber  20  is aligned with a cylindrical portion  25  projecting upward from the carburetor body  60 . A fixed metal fitting  22  for supporting an outer tube of a remote control cable is fitted into a tube  24  threadedly fitted on the cylindrical portion  25 . The metal fitting  22  is covered by a cap  21 . An inner wire (not shown) inserted into the outer tube is connected to the throttle valve  10  in a manner known in the art. 
     To bias the throttle valve toward its idle position, a return spring  30  is interposed between the tube  24  and the throttle valve  10 . To facilitate adjustment of the idle position of the throttle valve  10 , an inclined cam surface  62  is formed at the lower end of the throttle valve  10 . An idle adjustment bolt  35 , an O-ring  53 , and a spring  34  are fitted in a cylindrical bore  45  in the carburetor body  60 . A rod  32  extended from an end of the idle adjustment bolt  35  is threaded in a tapped hole  70  aligned with the bore  45 . The outer end of the rod  32  contacts the cam surface  62  when the throttle valve  10  is in its idle position. Hence, axial movement of the rod  32  varies the location of the engagement of the rod  32  with the cam surface  62  to control the idle position of the throttle valve  10 . A fuel adjustment needle valve  14  is carried by the throttle valve  10  and is fitted into a fuel supply pipe  13 . The fuel supply pipe  13  is received in the carburetor body  60  so that its upper end projects toward the valve chamber  20  to provide a main fuel nozzle. 
     A plate  27  is connected to the carburetor body  60  with a fuel pump diaphragm  6  sandwiched therebetween to form in part a fuel pump  28 . A pulsation pressure chamber  43  is defined in part by the diaphragm  6  and a passage  29  communicates crankcase pressure pulses with the chamber  43 . The pressure pulses displace the diaphragm  6 , and a pump chamber  5  on one side of the diaphragm  6  alternately expands and contracts. Fuel in a fuel tank, not shown, is taken into the pump chamber  5  via a fuel inlet pipe  37 , a passage  9 , a check valve  8  and a passage  7 , and further fed to a fuel metering chamber  46  via a passage  3 , a check valve  2 , and a fuel supply passage  12  including a valve chamber  51  accommodating an inlet valve  38 . The check valves  2  and  8  may be flaps formed integrally with diaphragm  6 . The fuel pump  28  as shown and described may be of conventional construction. 
     A fuel metering assembly  50  is also provided in the carburetor. An intermediate plate  39  is connected to the carburetor body  60  with a gasket  39   a  sandwiched therebetween. An end plate  40  is connected to the lower surface of the intermediate plate  39  with a fuel metering diaphragm  42  sandwiched therebetween. The fuel metering chamber  46  and an atmospheric chamber  41  are formed on opposite sides of the diaphragm  42 . A lever  44  is supported by a shaft  19  in the fuel metering chamber  46 . One end of the lever  44  is biased against a center projection  42   a  of the diaphragm  42 , and the other end of the lever  44  is engaged with a lower end of the inlet valve  38 . When the volume of the fuel metering chamber  46  is reduced, the diaphragm  42  is moved upwardly (as viewed in FIG. 1) by the intake vacuum pressure of the air intake passage  36 , the lever  44  rotates clockwise, the inlet valve  38  is opened, and fuel is supplied from the fuel pump  28  to the fuel metering chamber  46 . When the fuel metering chamber  46  is filled with fuel, the diaphragm  42  is pushed downwardly (as viewed in FIG. 1.) increasing the volume of the fuel metering chamber  46 , the lever  44  rotates counterclockwise and the inlet valve  38  engages a valve seat  33  to close and thereby stop fuel flow into the fuel metering chamber  46 . 
     As shown in FIG. 1, to communicate the fuel supply passage  12  with the air intake passage  36 , a bypass passage  16  is provided. In the embodiment shown, the bypass passage  16  is open at an inlet end to the valve chamber  51  (downstream of valve seat  33 ) and an orifice or restriction  48  is preferably provided generally adjacent to the fuel supply passage  12  at the inlet end of the bypass passage  16 . The restriction  48  suppresses excessive fluid flow from the valve chamber  51  to the bypass passage  16  to avoid undue interference with the operation of the fuel metering assembly  50 . The other end, or outlet end, of the bypass passage  16  opens into a connecting passage  17  that communicates the fuel metering chamber  46  with the air intake passage  36  through the fuel supply pipe  13 . A restriction  47  is preferably provided at an inlet of the connecting passage  17  between the fuel metering chamber  46  and the outlet end of the bypass passage  16 . 
     Preferably, the bypass passage  16  is routed around and does not communicate directly with the fuel metering chamber  46 . In the embodiment shown, the bypass passage communicates with the fuel supply passage in the area of the valve chamber  51 , which in turn is open to the fuel metering chamber  46 . At the other end, the bypass passage  16  communicates with the connecting passage  17 , which in turn is open to the fuel metering chamber  46  (through the restriction  47 ). Of course, other arrangements of the various chambers and passages, including the bypass passage  16 , may be employed. Also, while the bypass passage  16  is shown as being formed partly within the carburetor body  60  and partly outside the body  60 , such as by a tube or hose  16   a , the bypass passage  16  can be formed in other ways. For example, the bypass passage  16  can be formed entirely within the carburetor body  60  or entirely outside the body  60  as desired. Additionally, as used herein, the body of the carburetor includes main body  60  and the various plates (e.g. plates  27 ,  39 ,  40  in the embodiment shown) and bodies attached thereto. 
     When, for example, the operation of the engine is stopped and the engine is left in hot ambient conditions, such as out in the blazing sun, fuel vapor and/or air forms or remains in the pump chamber  5  of the fuel pump  18 , the fuel passage  12 , the inlet of the inlet valve  38  or the valve chamber  51 . This fuel vapor and/or air impairs or inhibits a smooth flow of fuel from the fuel pump  28  to the fuel metering chamber  46  at the re-start of the engine. Particularly, when the inlet valve  38  is opened, fuel vapor occurs at the inlet portion of the inlet valve  38  due to sudden lowering of fuel pressure. 
     Upon cranking of the engine to start it, vacuum pressure generated in the air intake passage  36  is communicated to the connecting passage  17  through the fuel supply pipe  13 . Fuel vapor or air in the clearance space between the valve housing  51  and the inlet valve  38  is taken into the air intake passage  36  via the restriction  48 , the bypass passage  16 , the connecting passage  17  and the fuel supply pipe  13 . In this manner, fuel vapor or air in the passage between the fuel pump  28  and the fuel metering chamber  46  is discharged into the air intake passage  36  quickly. With this air and fuel vapor removed from the carburetor, a more uniform flow of liquid fuel can be obtained from the fuel pump  28  to the fuel metering chamber  46 , and from the fuel metering chamber  46  to the air intake passage  36  (through the restriction  47 , the connecting passage  17  and the fuel supply pipe  13 ). Therefore, it is possible to suppress or prevent the flow of a large quantity of fuel vapor and/or air to the fuel metering chamber upon initial starting of the engine, providing a smoother start and initial idle operation of the engine. 
     With any large volume of fuel vapor and/or air removed from the fuel circuit, liquid fuel can promptly fill the passages between the fuel pump and inlet valve, and readily flow into the metering chamber to ensure its steady operation. Even if some fuel vapor and/or air remains, for example, in the clearance area between the valve chamber  51  and inlet valve  38 , this area is relatively small in volume and, in general, lesser quantities of fuel vapor and/or air do not significantly affect engine operation. It is noted that the present invention can be applied not only to the carburetor provided with a piston-type throttle valve, but also to carburetors provided with a barrel or other rotary throttle valve, or a butterfly-type throttle valve.