Abstract:
A fuel metering system for a combustion engine carburetor utilizes a non-convoluted, planar, flexible diaphragm which does not require a molding process to form a traditional convolution. The diaphragm defines in part a pressure controlled fuel metering chamber on one side and a reference chamber at atmospheric pressure on the other side. During operation of the engine, sub-atmospheric pressure within a fuel and air mixing passage draws fuel from the metering chamber to mix with air for combustion within the engine. As pressure within the metering chamber thus decreases, the diaphragm flexes into metering chamber. The displacement of the diaphragm actuates a flow control valve of the metering system which flows pressurized make-up fuel into the metering chamber until the diaphragm returns to its datum position. Preferably, hardware of the flow control valve which is in direct contact with a surface of the diaphragm exposed to the metering chamber does not penetrate the diaphragm as the traditional rivet and washer assembly would. Therefore, manufacturing costs are reduced and any opportunity of leakage between the fuel metering chamber and reference chamber is eliminated. Preferably, the carburetor is of a manual external purge type in order to exert sufficient vacuum within the metering chamber to displace the metering diaphragm thus opening the flow control valve to purge the carburetor of unwanted fuel vapor and air prior to starting the engine. The novel planar diaphragm thereby resolves problems associated with traditional metering diaphragms such as variation in convolution datum height affecting flow control valve lever/diaphragm clearances, non-symmetric convolution axis or distorted convolution affecting diaphragm pressure response and recovery.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of Ser. No. 09/650,166, filed Aug. 29, 2000 now U.S. Pat. No. 6,446,939. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a fuel metering system, and more particularly to a fuel metering system having a planar diaphragm for an externally-purged-type carburetor. 
     BACKGROUND OF THE INVENTION 
     Typically, carburetors have been used to supply a fuel-and-air mixture via an intake passage to both four stroke and two-stroke internal combustion engines. For many applications where small two-stroke engines are utilized, such as hand held power chain saws, weed trimmers, leaf blowers, garden equipment and the like, carburetors with both a diaphragm fuel delivery pump and diaphragm fuel metering system have been utilized. When the engine is operating, the diaphragm fuel delivery pump supplies fuel under pressure to the diaphragm fuel metering system through an inlet or flow control valve of the fuel metering system, which in-turn supplies fuel to a fuel-and-air mixing passage of the carburetor for mixing with air prior to flowing into a combustion cylinder of the engine. 
     A convoluted flexible diaphragm or membrane of the fuel metering system typically has a peripheral edge sealed to the carburetor body. A metering chamber and an air chamber is thus partitively disposed over and under the diaphragm, respectively. During operation, when the amount of fuel in the chamber decreases and the convoluted diaphragm is moved due to a negative pressure in the fuel-and-air mixing passage, the flow control valve is opened against the force of a spring by a pivoting lever that operates together with the diaphragm and is fixed to a wall of the carburetor body by a support shaft. In this way, the fuel is supplied from the fuel delivery pump to the metering chamber. As a result, the amount of fuel in the metering chamber is kept at about a constant level or volume. 
     Commonly, the carburetor has an external purge or manually actuated primer or suction pump having a flexible bulb attached to the bottom side of the carburetor body. The bulb internally defines a pump chamber in which a composite valve functions to admit fuel to the pump chamber and deliver fuel to the metering chamber of the fuel metering system. Moreover, before the engine starts for operation, the bulb is repetitively manually pressed and released to suck unwanted fuel vapor and air from the fuel pump and fuel metering system into the pump chamber of the external purge via the composite valve. The fuel vapor and air are transferred back to the fuel tank via the composite valve. At this time, since the metering chamber is under a negative pressure, the fuel in the fuel tank is supplied to the metering chamber through a fuel chamber of the fuel delivery pump and the flow control valve. 
     The diaphragm of the fuel metering system typically has five basic functions: (1) maintain a seal between the air and the metering chambers, (2) respond instantly to differential pressure (engine manifold pressure referenced to atmospheric), (3) open the flow control valve when the engine needs fuel, (4) close the flow control valve when the engine has enough fuel, and (5) perform consistently over the life of the engine (i.e., no loss of elastomeric flexibility of the convoluted diaphragm from age or fuel exposure). 
     The convoluted metering diaphragm is typically made of an elastomeric membrane and molded to form convolutions to achieve flexibility and a pre-established total travel distance necessary to open and close the flow control valve. This total travel distance commonly ranges from about 0.020 to 0.065 of an inch, and includes a degree of free-play before a head of the flow control valve actually moves to open and close the valve. During engine operation, from idle to wide open throttle conditions, the convoluted diaphragm typically moves approximately within a range of 0.001 to 0.015 of an inch and thus the head proportionately moves accordingly. This range depends upon the carburetor and its application. FIGS. 8-10, illustrated as prior art, show such a metering diaphragm  20  having a molded convolution  22 . Under normal engine/carburetor operating conditions, a center or circular section  24  of the diaphragm, circumscribed by the convolution  22 , provides the primary movement for operation of the flow control valve  26 . The convolution itself has little contribution to achieving the required fuel delivery pressure balance in the metering chamber (not shown). The metering diaphragm  20  transmits a relative movement to a pivoting lever  28  which transmits opposite movement to a head  30  of the flow control valve  26  based on a pressure differential formed across the diaphragm. The differential is initiated from the sub-atmospheric pressure exposed to the metering chamber by the fuel-and-air mixing passage of the carburetor and the reference atmospheric pressure of the air chamber of the metering system. 
     FIGS. 8 and 9 illustrate the common convoluted metering diaphragm  20  having a central rigid plate  32 , a washer  34  and a rivet button  36  for transmitting this force to the pivoting and spring biased lever  28  of the flow control valve  26 , which in turn moves the valve head  30  away from a valve seat  38  carried by the carburetor body to open, and against the valve seat  38  via the resilience of the spring (not shown) to close the valve. The diaphragm must have sufficient resilience for transmitting displacement in proportion to the pressure differential, yet remain flexible enough to respond to sudden changes in pressure such as for engine acceleration and engine starting. Unfortunately, the cost of manufacturing a flexible diaphragm having rigid hardware which is engaged sealably to the diaphragm is expensive, and the diaphragm penetration required to secure the hardware creates a source of potential leakage between the metering chamber and the reference chamber. 
     Aside from the rigid hardware, there are several reasons for the additional diaphragm travel afforded by the convolution in a standard diaphragm carburetor design. The convolution provides extra material for maintaining diaphragm flexibility should the fabric or elastomer coating shrink (typically made of woven silk and nitrile material) upon exposure to hydrocarbon fuels or aging effect. This extra material measured or extending perpendicular to the general plan of the diaphragm itself also maintains necessary operating clearances or free-play travel distance between the pivoting lever and diaphragm if this shrinkage occurs. The extra convolution material also allows more diaphragm travel (increased metering fork leverage) to “uncork” a stuck head of the flow control valve, particularly for carburetors which do not have a manual external purge or bulb device to create a strong vacuum. In-other-words, the convolution assists to release stuck heads for those carburetors which utilize the weaker engine manifold vacuum in combination with a choke valve to generate the metering chamber vacuum for opening the flow control valve for purging the carburetor of air or vapor to better start the engine. 
     However, there are also inherent problems associated with the metering diaphragm convolution which have adverse impact on carburetor performance. Such problems include the inadvertent changes in baseline carburetor fuel flow settings, inconsistent fuel delivery and exhaust emission variation, poor acceleration response, and the potential for leaking/dripping from the carburetor main nozzle. For instance, a distorted convoluted diaphragm can change the original or installed operating clearance between the rivet button and the lever so that an adverse shift in idle performance due to vibration or orientation of the engine can cause fuel leakage leading to a rich idling engine. At wide open throttle conditions, such fuel leakage can result in engine stall during deceleration from wide open throttle to idle. For non-running engines, a distorted convolution which eliminates clearance can depress the lever to allow fuel leakage out of the carburetor causing fuel tank drainage. 
     The process of convolution molding is known to contribute to variations in diaphragm flexibility based on molding temperatures and pressures, and aging which is also influenced by the composition of the elastomeric material and substrate fibers. Natural cotton or silk substrates have been used historically for flexibility and elastomeric bonding, but these natural fibers in combination with a molded convolution are susceptible to hygroscopic absorption leading to uncontrolled changes in convolution height influenced by ambient humidity which directly adversely impacts the operating clearance. Use of nylon or other synthetic polymers in lieu of natural fibers as the substrate material for the molding process to create the convolution may contribute to additional molding stress and memory set of the convolution resulting in diaphragm rigidity and inconsistent response to small differential pressures. Thickness variation of the elastomeric coating and its cured state also contribute to poor diaphragm response and flexibility changes through molding the metering diaphragm convolution. Pin holes or elastomer tears can occur at the base of the convolution during the molding process where the base material is squeezed and stretched under heat and pressure, leading to potential fuel and/or air leaks across the metering diaphragm. 
     In addition, residual stresses from both the molding process and fabrication of the diaphragm material can be accentuated upon exposure to hydrocarbon and aromatic compounds in the fuel causing diaphragm convolution distortion or changes in material property. For example, conventional Nitrile rubber compounds can lose plasticicizers blended in the rubber from fuel leachment breaking the elastomeric chemical bonds resulting in adverse stiffness affecting flexibility characteristics of the convoluted metering diaphragm. Other types of elastomeric and substrate materials may also exhibit various degrees of swell, shrinkage, and flexibility characteristics exacerbated by the convolution which alter the ability of the diaphragm to respond consistently and repeatably to small pressure differentials. 
     Specific convolution anomalies involving convoluted metering diaphragms include variation in convolution datum height affecting lever/diaphragm clearances, non-symmetric convolution axis or distorted convolution affecting diaphragm pressure response and recovery, oil canning of the diaphragm during flexure causing erratic diaphragm movement, fuel and air leakage across the diaphragm from holes or tears or poor elastomeric coating processes. These examples contribute inconsistent carburetor fuel flow settings, poor engine acceleration, engine stalls during rollout, hard starting, and fuel leakage/flooding. It becomes more of a prevalent problem on those engine applications with relative weak manifold vacuum, lean carburetor setting for lower exhaust emissions, or large frictional differences in the engine (new versus broke-in engine) which make the carburetor more sensitive to variation in diaphragm flexibility. 
     SUMMARY OF THE INVENTION 
     A fuel metering system for a combustion engine carburetor utilizes a non-convoluted, planar, flexible diaphragm which does not require a molding process to form a traditional convolution. The diaphragm defines in part a fuel metering chamber on one side and a reference chamber at near atmospheric pressure on the other side. During operation of the engine, sub-atmospheric pressure within a fuel-and-air mixing passage draws fuel from the metering chamber to mix with air for combustion within the engine. As pressure within the metering chamber thus decreases, the diaphragm flexes into metering chamber. The displacement of the diaphragm actuates a flow control valve of the metering system which flows pressurized make-up fuel into the metering chamber until the diaphragm returns to its datum position. Preferably, hardware of the flow control valve which is in direct contact with a surface of the diaphragm exposed to the metering chamber does not require penetration of the diaphragm, as the traditional rivet and washer assembly does. Therefore, manufacturing costs are reduced and any opportunity of leakage between the fuel metering chamber and reference chamber is eliminated. Preferably, the carburetor is of a manual external purge type in order to exert sufficient vacuum within the metering chamber to displace the planar metering diaphragm thus opening the flow control valve to purge the carburetor of unwanted fuel vapor and air prior to starting the engine. The novel planar diaphragm thereby resolves problems associated with traditional convoluted metering diaphragms such as the variation in convolution datum height affecting flow control valve lever/diaphragm clearances, and non-symmetric convolution axis or distorted convolution affecting diaphragm pressure response and recovery. 
     Preferably, in order to achieve the flexibility and fuel absorption resistance necessary for the unique operating characteristics of the flat metering diaphragm, the traditional composite material of nitrile and silk fabric is replaced with a a synthetic woven fabric impregnated with a synthetic rubber, such as nylon and nitrile. The nylon fabric has extremely small diameter fiber bundles in the weave providing increased flexibility with favorable recovery characteristics (return to datum position upon removal of differential pressure across the diaphragm). In addition, the elastomeric composition is such that fuel permeability is decreased when compared to that of typical diaphragm materials used in the past. This decrease in fuel permeability is favorable for emission control requirements. Moreover, the synthetic rubber and fabric combination preferably has a surface texture and elastomeric properties conducive to minimal abrasion wear. This is necessary for the preferable novel flow control valve lever of the present invention which must act directly upon the metering diaphragm in both wet and dry environments. 
     Objects, features and advantages of this invention include a metering diaphragm which is non-convoluted eliminating the convolution height variations created in manufacturing, diaphragm fuel absorption and aging of the traditional diaphragm which adversely affects flow control valve and thus engine operation. Moreover, leakage between the metering and air chamber is eliminated via the novel flow control valve lever of the present invention thereby providing a reliable smooth running engine. Additional advantages are a reduced number of parts, reduced number of manufacturing processes, and a design which is easily incorporated into existing carburetors. This design improves engine performance and is relatively simple and economical to manufacture and assemble, and in service has a significantly increased useful life. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of this invention will be apparent from the following detailed description, appended claims, and accompanying drawings in which: 
     FIG. 1 is a cross-section of an externally purged, butterfly valve type, carburetor having a fuel metering system of the present invention; 
     FIG. 2 is a plan view of the planar metering diaphragm; 
     FIG. 3 is an enlarged partial cross-section of the planar metering diaphragm taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is a cross-section of an externally purged, rotary type, carburetor having a second embodiment of a fuel metering system; 
     FIG. 5 is a top view of a lever of the second embodiment of the fuel metering system; 
     FIG. 6 is a cross-section of the lever taken along line  6 — 6  of FIG. 5; 
     FIG. 7 is a bottom view of the lever; 
     FIG. 8 is a partial side view of a prior art fuel metering system; 
     FIG. 9 is a plan view of a convoluted metering diaphragm of the prior art fuel metering system; and 
     FIG. 10 is a cross-section of the convoluted metering diaphragm taken along line  10 — 10  of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring in more detail to the drawings, FIG. 1 illustrates a carburetor  40  according to a first embodiment of the present invention which is of a butterfly valve type. Carburetor  40  has a main body  42  through which a fuel and air mixing passage  44  extends. A fuel metering system  46  carried by the body  42  delivers fuel at a controlled pressure to the fuel and air mixing passage  44  and receives fuel through a flow control valve  48  from a fuel pump  50 , also carried by the carburetor body. A purge pump assembly  52  is generally mounted externally to the carburetor body for the manual purging of fuel vapor and air from the fuel metering system  46 , the fuel pump  50  and associated passages to assist in reliable starting of the engine. 
     A pressure pulse passage  54  defined by the carburetor body  42  communicates at one end with a crankcase of the engine (not shown) and opens at the other end to a pressure pulse chamber  56  of the fuel pump  50 . The fuel pump  50  has a flexible diaphragm  58  engaged sealably to the carburetor body  42  generally along a peripheral edge  60 . The fuel pump diaphragm  58  defines in part a fuel pump chamber  62  on one side and the pressure pulse chamber  56  on its other side and is displaceable in response to a difference in pressure between the chambers  56 ,  62 . 
     When the engine is running, pressure pulses from its crankcase are directed to the pressure pulse chamber  56  via the pressure pulse passage  54 . When a negative pressure pulse is transmitted to the pulse chamber  56 , the flexible fuel pump diaphragm  58  is moved in a direction increasing the volume of the fuel pump chamber  62  and decreasing the volume of the pressure pulse chamber  56 . The increase in the fuel pump chamber volume draws fuel from a fuel pump reservoir or tank (not shown) through an inlet nozzle  64  formed in the carburetor body  42 , and through an inlet passage  66  which communicates with the fuel pump chamber  62  and is interposed by an inlet valve  68 . The inlet valve  68  controls fluid flow through the inlet passage  66  to the fuel pump chamber  62  and is preferably a flap type valve integral with the diaphragm  60  and adapted to selectively engage a valve seat  70  carried by the body  42  in order to close. The pressure drop caused by the increase in volume of the fuel pump chamber  62  causes the inlet valve  68  to open and to permit fuel to flow from the inlet nozzle  64  to the fuel pump chamber  62 . 
     During the engine cycle, as the pressure in the engine crankcase is increased, a positive pressure pulse will be transmitted through the crankcase pressure pulse passage  54  to the pressure pulse chamber  56  to cause the diaphragm  58  to move in a direction decreasing the volume of the fuel pump chamber  62  and increasing the volume of the pressure pulse chamber  56 . The decrease in volume of the fuel pump chamber  62  increases the pressure therein and thereby closes the inlet valve  68  and forces fuel in the fuel pump chamber  62  toward an outlet passage  72  which is interposed by an outlet valve  74 . The outlet valve  74  is also preferably a flap type valve integral with the diaphragm  58  and adapted to selectively engage a valve seat  76  to close the outlet passage  72 . When a negative pressure condition exists in the fuel pump chamber  62 , the outlet valve  74  is closed and a positive pressure in the fuel pump chamber  62  opens the outlet valve  74  to permit the fuel to be subsequently delivered from the fuel pump chamber  62  to the downstream fuel metering system  46 . A fuel filter  78  such as a screen or other porous member is preferably disposed across the outlet passage  72  within the body  42 . 
     Fuel which passes through the fuel filter  78  enters a fuel metering inlet passage  80  and is delivered under pressure to the fuel metering system  46  of the carburetor  40 . The fuel metering system  46  functions as a pressure regulator receiving pressurized fuel from the fuel pump  50  and regulating its pressure to a predetermined pressure, usually sub-atmospheric, to control the delivery of the fuel from the fuel metering system  46 . The fuel metering inlet passage  80  provides fuel to a fuel metering chamber  84  of the fuel metering system  46 . The flow control valve  48  operatively obstructs the inlet passage  80  to selectively permit fuel flow from the inlet passage  80  to the fuel metering chamber  84 . The flow control valve  48  has a valve body  86 , a generally conical valve head  88  extending from the body and engageable with an annular valve seat  90  which defines the inlet of the fuel metering chamber  84 , and a needle  92  extending through the valve seat  90  and into the fuel metering chamber  84 . A spring  94  bears on the end of the body  86  opposite the needle  92  to yieldably bias the valve  48  to its closed position with the valve head  88  bearing on the valve seat  90  to prevent fuel flow into the fuel metering chamber  84 . At its other end, the spring  94  bears on an adjustment member embodied as a screw  96  received in a threaded bore  98  through the carburetor body  42 . The position of the screw  96  in the bore  98  can be adjusted to adjust the working length of the spring  94  and hence, the spring force acting on the flow control valve  48  to change the operating characteristics of the valve. 
     The fuel metering chamber  84  is defined in part by the carburetor body  42  and by a first side  99  of a flexible planar diaphragm  100  sealed along a periphery  102  by the body. The fuel metering chamber  84  also has a fuel outlet port  104  through which fuel is discharged to be delivered to the engine, and a purge outlet passage  106  interposed by a check valve  108  to permit fluid flow therethrough only when the purge pump assembly  52  is actuated to facilitate removing any fuel vapor or air from the fuel metering chamber  84  and filling it with liquid fuel prior to initial operation of the engine. On an opposite second side  109  of the planar fuel metering diaphragm  100 , an air or reference chamber  110  is defined in part by the body  42 . The air chamber  110  is maintained at substantially atmospheric pressure by a vent  112  in the chamber  110  which communicates with an atmospheric pressure source, such as the exterior of the carburetor. A substantially rigid disk  114  is disposed in the fuel metering chamber  84  between the planar fuel metering diaphragm  100  and one or more fixed pivots  116  extending from the carburetor body  42  into the fuel metering chamber  84 . The disk  114  extends from the fixed pivot points  116  and underlies the needle  92  of the flow control valve  48 . 
     Fuel flows out of the metering chamber fuel outlet port  104  in response to pressure pulses produced in an engine intake manifold which propagate through the fuel and air mixing passage  44 , through a fuel flow control assembly  118  and to the fuel metering chamber  84 . A negative pressure pulse transmitted to the fuel metering chamber  84  draws fuel out of the metering chamber fuel outlet port  104  creating a pressure differential between the fuel metering chamber  84  and the air chamber  110 . This pressure differential across the fuel metering diaphragm  100  causes the diaphragm  100  to move in a direction tending to decrease the volume of the fuel metering chamber  84  and increase the volume of the air chamber  110 . 
     This movement of the planar fuel metering diaphragm  100  moves the disk  114  in a similar direction. Movement of the disk  114  causes it to engage the fixed pivots  116  along one side which tends to rock or pivot the disk  114  into engagement with the needle  92  of the flow control valve  48  at its opposite side. As the pressure differential between the metering chamber  84  and the air chamber  110  increases, the force exerted on the disk  114  by the diaphragm  100  is eventually sufficient to displace the flow control valve  48  to an open position permitting flow of the pressurized fuel in the inlet passage  80  to the fuel pump metering chamber  84 . As the pressurized fuel enters the fuel metering chamber  84 , the pressure therein increases thereby reducing the pressure differential across the planar diaphragm  100 . Likewise, the force exerted on the disk  114  by the diaphragm  100  is then decreased until eventually the force is insufficient to overcome the force biasing the flow control valve  48  to its closed position whereby the flow control valve closes and the flow of fuel into the fuel metering chamber  84  is prevented. In this manner, the flow control valve  48  is continuously cycled between open and closed positions in response to the pressure differential across the planar fuel metering diaphragm  100  to maintain the fuel in the metering chamber  84  at a constant average pressure relative to the pressure in the air chamber  110 . Notably, because a negative pressure pulse from the intake manifold is used to actuate the fuel metering diaphragm  100 , the average pressure in the fuel metering chamber  84  is at least slightly sub atmospheric. 
     Fuel discharged from the fuel metering chamber fuel outlet port  104  flows into a main fuel delivery passage  118 . The main fuel delivery passage  118  leads to an adjustable low speed needle valve  120  and an adjustable high speed needle valve  122  downstream of the low speed needle valve. Each needle valve  120 ,  122  is of generally conventional construction arranged to adjustably obstruct respective low and high speed fuel passages  124 ,  126  which branch off downstream from the main fuel delivery passage  118 . Fuel which flows through the low speed fuel delivery passage  124  leads to a plurality of conventional fuel jets  128  communicating with the fuel and air mixing passage  44  near a butterfly throttle valve  130 . Fuel which flows through the high speed fuel delivery passage  126  enters a high speed fuel nozzle  132  which is open to the fuel and air mixing passage  44  at a venture  133  of the mixing passage. The high speed fuel nozzle  132  may comprise a restriction or nozzle disposed in a portion of the high speed fuel delivery passage  126 . 
     The fuel and air mixing passage  44  has a venturi portion  134  upstream of the throttle valve  130  received in the passage  44 . The throttle valve  130  is movable from an idle position substantially closing the fuel and air mixing passage  44  to limit the fluid flow therethrough, to a wide open position generally parallel with the axis of the passage  44  to permit a substantially unrestricted fluid flow therethrough. The plurality of fuel jets  128  comprise a primary fuel jet  136  disposed downstream of the throttle valve  130  when it is in its closed position and one or more secondary fuel jets  138  disposed upstream of the throttle valve  130  when it is in its closed position. More or less than the number of primary and secondary fuel jets  128  shown may be used as desired for a particular application. 
     Fuel flows from the fuel metering chamber  84  through the main fuel delivery passage  118 , the fuel needle valves  120 ,  122  and eventually to the idle fuel jets  128  and high speed fuel nozzle  132  in response to the manifold pressure signals as previously mentioned. As shown in FIG. 1, during engine idle operating conditions, the throttle valve  130  is in its idle position substantially closing the fuel and air mixing passage  44 . The manifold negative pressure signal is prevented from reaching the high speed fuel nozzle  132  by the throttle valve  130 . Thus, there is no fuel flow past the high speed needle valve  122  because there is little or no pressure drop across the high speed fuel nozzle  132  to induce a flow through the high speed fuel delivery passage  126 . 
     At idle, fuel flow required to operate the engine is supplied through the low speed fuel delivery passage  124 . However, the secondary fuel jets  138  are not exposed to the manifold vacuum signal due to their position upstream to the throttle valve  130  when it is in its idle position. Rather, air flowing through the fuel-and-air mixing passage  44  bleeds through the secondary fuel jets  138  into a progression pocket portion  139  of the passage  124  providing a fuel-and-air mixture within the progression pocket portion  139 . Air flow from the fuel-and-air mixing passage  44  through the high speed fuel delivery passage  126  is preferably prevented by a check valve  140  to control the quantity of air provided to progression pocket portion of the low speed fuel passage  124 . The primary fuel jet  136  is exposed to the manifold vacuum signal and hence, the fuel and air mixture within the low-speed fuel passage  124  is drawn through the primary fuel jet  136  into the fuel-and-air mixing passage  44  whereupon it is combined with the air flowing through the passage  44  to be delivered to the engine. Therefore, at engine idle operating conditions all the fuel delivered to the engine is supplied through the primary fuel jet  136 . The air bleed through the secondary fuel jets  138  is desirable to provide air into the progression pocket portion  139  and thereby reduce the rate at which liquid fuel is drawn through the primary fuel jet  136  in use. If the secondary fuel jets  138  were not present and air was not provided into the progression pocket portion  139 , too much liquid fuel would flow through the primary fuel jet  136  if it were maintained the same size, or in the alternative, a much smaller and much harder to manufacture primary fuel jet would be required to provide the proper liquid fuel flow rate to operate the engine properly at idle operating conditions. 
     As the throttle valve  130  is rotated from its idle position to its wide open position to increase engine speed, the manifold vacuum from the engine is increasingly exposed to the secondary fuel jets  138 . At some point during the throttle valve opening, the negative pressure or pressure drop across the secondary fuel jets  138  becomes great enough such that air is no longer fed from the fuel-and-air mixing passage  44  into the progression pocket portion  139  but rather, fuel in the progression pocket is drawn through the secondary fuel jets  138  into the fuel and air mixing passage  44 . The size and spacing of the primary fuel jet  136  and each of the secondary fuel jets  138  in relationship to each other and the throttle valve  130  is very important to the proper operation of a specific engine to ensure that the desired fuel and air mixture is supplied to the engine during its wide range of operating conditions. 
     When the throttle valve  130  is opened further to its wide open position, the engine manifold vacuum signal reaches the venturi  133  and the high speed fuel nozzle  132  creating a pressure drop across the fuel nozzle  132  and drawing fuel therethrough to be mixed with air flowing through the fuel and air mixing passage  44 . Air flow through the venturi  133  also creates a pressure drop across the high speed fuel nozzle  132  to increase the fuel drawn therethrough. The increased vacuum across the high speed fuel nozzle  132  provides an increased flow of fuel through the high speed fuel nozzle which is required for good engine acceleration when the throttle valve  130  is quickly opened from its idle position to its wide open position. The flow area and position of the high speed fuel nozzle  132  relative to the throttle valve  130  and the venturi  133  is important to ensure the desired fuel and air mixture is provided to the engine. At wide open throttle engine operating conditions, a portion of the fuel is also preferably delivered from the fuel jets  128  in addition to that supplied through the high speed fuel nozzle  132 . 
     The air purge assembly  52  is used to prime the carburetor  40  to ensure that liquid fuel is present in all passages from the fuel reservoir to the fuel metering chamber  84  and to remove air and fuel vapor therefrom before the engine is started. This greatly reduces the number of engine revolutions required to start the engine. The air purge assembly  52  comprises a flexible bulb  142  having a radially outwardly extending rim  144  trapped between a cover  146  and the bottom of the carburetor body  42  defining a bulb chamber  148 , an air purge inlet passage  150  extending from the purge outlet passage  106  of the fuel metering chamber  84  to the bulb chamber  148 , and an air purge outlet passage  152  leading from the bulb chamber  148  to a purge outlet nozzle  154  leading to a fuel reservoir through which fluid pumped out of the carburetor  40  is discharged to the reservoir. A check valve  156  closes the air purge outlet passage  152  until a sufficient pressure within the bulb chamber  148  displaces the check valve  156  to permit fluid flow therethrough into the reservoir. Similarly, the check valve  108  closes the purge outlet passage  106  of the fuel metering chamber  84  to prevent fluid flow from the bulb chamber  148  to the fuel metering chamber  84  when the bulb is depressed and to permit fluid flow out of the fuel metering chamber  84  to the bulb chamber  148  only when a sufficient pressure differential exists across the check valve  108  to open it against the bias of a spring tending to close it. 
     The air purge process is initiated by depressing the bulb  142  which pushes the air, fuel vapor and/or fuel within the bulb chamber  148  through the outlet passage check valve  156  and the outlet passage  152  back to the fuel reservoir. The check valve  108  at the outlet passage  106  prevents any fluid from being pushed into the fuel metering chamber  84 . When the bulb  142  is released, the volume of the bulb chamber  148  increases creating a vacuum because the outlet check valve  156  does not permit fluid flow back into the bulb chamber  148 . The vacuum is transmitted through the air purge inlet passage  150  to the check valve  108  disposed within the outlet passage  106 . The spring biasing this check valve  108  determines the magnitude or force of the vacuum required to open it and permit fluid in the metering chamber  84  to flow through the air purge inlet passage  150  to the bulb chamber  148 . This check valve spring also adds an extra force to the check valve  108  relative to the negative pressure prevailing within the fuel metering chamber  84  during engine operation, to ensure a good seal between the metering chamber  84  and air purge inlet passage  150  to prevent fluid leakage from the fuel metering chamber during all engine operating conditions (exclusive of the air purge process). When the vacuum at the check valve  108  is sufficient to open it, fluid and air within the fuel metering chamber  84  is drawn through the air purge inlet passage  150  into the bulb chamber  186 . Subsequent depression of the bulb  142  then forces this fluid and air through the check valve  156  and the outlet passage  152  to the fuel reservoir. 
     A manual external purge, such as that of the external purge assembly  52 , is preferable over other purge devices, such as an automatic choke previously described, because the vacuum transmitted to the fuel metering chamber  84  during the manual purge process is particularly strong and thus capable of displacing the planar diaphragm  104 , whereas the common convoluted diaphragm requires less vacuum to cause equal displacement. This displacement created by the strong vacuum when the check valve  108  is open also displaces the disk  114  toward the flow control valve  48  to open it and thereby draw fuel through the fuel pump  50 , the fuel metering inlet passage  80  and into the fuel metering chamber  84  to fill them all with liquid fuel. A check valve  158  at the fuel outlet  104  of the fuel metering chamber  84  is closed by the application of the air purge vacuum to the fuel metering chamber  84  to prevent air from being pulled from the fuel and air mixing passage  44 , through the fuel jets  128  and fuel delivery passages  124 ,  126 ,  118  into the fuel metering chamber  84 . Several actuations or depressions of the bulb  142  may be necessary to draw fuel from the reservoir, through the fuel pump  50  and fuel metering system  46  and finally into the bulb chamber  148 . The number of actuations of the bulb  142  required is a function of the volume of the bulb chamber  148  compared to the volume of the passages that lead from the fuel reservoir to the bulb chamber. 
     The flat disk  114  within the fuel metering chamber  84 , used to actuate the flow control valve  48 , eliminates many of the pockets or cavities required in conventional carburetors to accommodate the levers, inlet valve and a spring biasing the valve lever. Each of these cavities in a conventional carburetor creates a discontinuous surface of the carburetor body in which fuel vapor can collect and coalesce until eventually it is drawn through the fuel passages of the carburetor and delivered to the engine providing a temporarily lean fuel and air mixture to the engine which is undesirable. Further, with the flat disk  144  on the fuel metering diaphragm  100 , no holes or openings need be formed through the fuel metering diaphragm  100  as in prior carburetors thereby simplifying its manufacture and assembly into the carburetor and increasing its in service useful life. Desirably, capillary forces between the disk  114  and the wet fuel metering diaphragm  100  are sufficient under normal operating conditions to maintain the disk  114  in contact with the diaphragm  100  so that the disk  114  moves with the diaphragm to actuate the flow control valve  48 . Therefore, the disk  114  not only provides a simpler lever or actuating mechanism for the flow control valve  48 , it also eliminates a number of the pockets in which fuel vapor collects in conventional carburetors. 
     Referring to FIGS. 2-3, the fuel metering diaphragm  100  is substantially flat and without convolutions thereby eliminating the unpredictable fuel metering variation caused by unpredictable clearance variations between the convoluted diaphragm and associated fuel flow control valves. Flat diaphragms also reduce manufacturing costs by eliminating the molding process necessary to produce the convolution. Because the vertical or lateral travel of the flat diaphragm  100  is more exact than that of a convoluted diaphragm, its vertical travel can be minimized while maintaining necessary response of the associated flow control valve  48 . This reduced travel of the flat diaphragm  100  improves engine start at elevated ambient temperatures of approximately greater than 90° Fahrenheit or engine start of engines having heated carburetors from prior running periods. This is so because heated liquid fuel disposed downstream at the flow control valve  48  is more susceptible to vapor generation and flash-off of the lighter aromatic constituents. The reduced travel of the flat diaphragm  100  during initial engine start does not move the head  86  of the flow control valve  48  as much as a conventional convoluted diaphragm would. Therefore, for each attempted start of the engine, the head  86  will remain seated or partially restricted permitting less fuel vapor ingestion into the metering chamber  84  during each start attempt. After the engine has started, the fuel delivery pump  50  generates fuel pressure suppressing vapor formation. 
     The fuel metering diaphragm  100  is preferably a woven synthetic fabric  160 , such as nylon, impregnated or layered with an elastomeric coating forming a sheet or a homogeneous thin film polymeric material, and is thus flexible to move in response to a differential pressure across it without the need for the convolution. Also preferably, the diaphragm  100  is formed of a material that swells when exposed to liquid fuel to increase its flexibility and responsiveness. A swell of 2% to 10% is desirable because it increases the flexibility of the diaphragm without having to artificially stretch the diaphragm which makes assembly difficult. Other currently preferred composite materials for the fuel metering diaphragm are mylar/kapton or a high density polyethylene because the materials have excellent flexibility, strength, is resistant to degradation in fuel and resists developing a static charge. The diaphragm is preferably between 0.5 to 2 mil. thick. One specific composite sheet, suitable for a flat fuel diaphragm application, is that made by ContiTech North America, Inc. Montvale, N.J., identified as model number 23-009, made of generally nitrile rubber and woven nylon having a thickness of approximately 0.18 millimeters. Other polymers may also be used such as, for example, linear low density polyethylene, low density polyethylene, fluoroelastomer, fluorosilicone, chlorotrifluoroethylene copolymers, polyvinylidene fluoride, polyvinyl fluoride, polyamide, polyether ether keytone, fluorinated ethylene propylene, and microthin metals such as stainless steel without the use of a woven fabric to name a few. The conventional composite material of woven silk fabric impregnated with nitril for convoluted diaphragms is not preferred for flat diaphragms because this material when fuel soaked stretches too much thus providing little pull to return the diaphragm to its original shape. 
     Referring to FIGS. 4-7, a second embodiment of a carburetor  40 ′ is illustrated utilizing a flat fuel metering diaphragm  100 ′. Carburetor  40 ′ is shown as a rotary-type having a manual external purge assembly  52 ′ which utilizes a duck bill type check valve  156 ′ performing the combined functions of metering check valve  108  and purge check valve  156  of the first embodiment. 
     Of particular interest is the fuel metering system  46 ′ which eliminates the rigid disk  114  of the first embodiment and replaces it with a pivoting lever  114 ′, best shown in FIGS. 5-7. Lever  114 ′ operates similar to lever  28  previously described and illustrated in FIG.  8 . However, for a flat diaphragm application, the common rivet  36 , washer  34 , and plate  32  are not required. Instead, a non-abrasive convex surface  164  of an end or end cup portion  166  of the lever  114 ′ rides directly against an approximate central point of the flat diaphragm  100 ′. A second opposite end  168  of the elongated lever  114 ′ is fork-like in shape opening along the lever&#39;s longitude to operatively engage an end portion of a head of the flow control valve (not shown). An elongated hole or passage  170  is carried by and extends laterally through the lever  114 ′ and snugly receives a rod (not shown) engaged rigidly to the carburetor body and about which the lever pivots. Lever  28  of the prior art has typically been made of aluminum which permits bending of the lever itself within the manufacturing process to adjust for variations in clearance and tolerance of the convolution  22  of the diaphragm  20  if applied, and the flow control valve hardware. Because such variations do not exist with the flat diaphragm  100 ′, as oppose to a convoluted one, the bending operation may be eliminated permitting manufacturing of the non-abrasive lever  114 ′ as a preferable one-piece injection molded plastic part preferably made of a nylon or acetal material. 
     While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. It is not intended herein to mention all the possible equivalent forms or ramification of the invention. It is understood that terms used herein are merely descriptive, rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention.