Abstract:
A carburetor fuel adjustment assembly includes a low speed needle valve and preferably a high speed needle valve threaded into respective receptacles of a carburetor body. Each receptacle defines an elongated cavity which intersects a fuel passage and has a valve orifice or seat. Each needle valve has a shank which threadably engages the respective receptacles and a tip that extends into the axially-aligned orifice or seat. The tip is axially movable relative to the orifice by rotation of the needle valve to control the size of the opening between the valve and orifice for fuel flow. A resilient body cooperates with the valve shank in laterally biasing the tip into a steadfast position relative to the orifice or seat. The lateral bias assures constant area for fuel flow through the orifice by resisting needle movement until a sufficient torque is intentionally applied to the needle valve.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to a carburetor and more particularly to a fuel flow adjustment assembly of a carburetor for a combustion engine. 
     BACKGROUND OF THE INVENTION 
     As shown in  FIGS. 1 and 2 , a known prior art carburetor has a fuel adjustment assembly  16  with low and high speed adjustable needle valves  17 , each threaded into a needle valve receptacle  18  in a carburetor body  19 . To permit adjustment of fuel flow, each valve receptacle  18  communicates with a separate fuel passage (not shown) in the carburetor body. Each needle valve  17  generally includes a distal tip  21 , an enlarged head  22  and a threaded shank  23  disposed between the tip and the enlarged head. The threaded shank  23  of the needle valve  17  engages a female threaded portion  20  of the needle valve receptacle  18 . The tip  21  of the valve  17  may be positioned within an axially-aligned needle seat orifice of the fuel passage and can be axially advanced and retracted, by rotation of the needle valve  17 , to adjust the fuel flow rate. Axial advancement and retraction of the distal tip  21  in the seat orifice respectively decreases and increases the amount of fuel that can flow through the orifice by decreasing and increasing the cross-sectional area through which fuel flows. The enlarged head  22  of the needle valve  17  is rotated by using a tool such as a screwdriver inserted into a diametric slot  24  in the head  22  which protrudes from the carburetor body  19 . In some such assemblies, to prevent inadvertent or un-commanded rotation of the needle valve  17 , an adjustment needle limiter cap  25  is placed over the screw head  22  and is engagable with an adjacent stop. 
     Because of machining tolerances and limitations during manufacture, fuel adjustment assemblies of this type include enough clearance between the threads of shank  23  of the needle valve  17  and the valve receptacle  18  to allow for lateral and axial movement of the tip  21  relative to the needle seat orifice when force is applied to the valve head  22 . This lateral and axial movement can change the size of the orifice flow area enough to result in fuel flow rate changes of up to 20% from an optimum fuel flow rate determined by the manufacturer. Fuel flow rate changes caused by this needle “slop or wobble” result in excessively rich or lean fuel mixtures that undesirably increase exhaust emissions or affect engine performance. Therefore, it is desirable to reduce fuel flow fluctuations through the needle valve  17  and the resulting increase in exhaust emissions and/or deterioration of engine performance by limiting needle slop and wobble. 
     Not only is it desirable to limit or hold steady the lateral and axial position of the needle valve tip  21  with respect to the orifice and regardless of the valve&#39;s rotational position, it is also desirable to maintain the desired setting of the fuel flow in a running engine. Any inadvertent rotation of the needle valve  17 , possibly caused by the vibration of a running engine or placement of a conventional limiter cap  25  over the valve&#39;s head  22  and after valve adjustments, can alter desired setting of the fuel flow. Therefore it is desirable to restrain the rotation of the needle valve  17  thereby preventing any unintended changes to the fuel flow setting. To do so, traditionally, compression springs  26  are disposed concentrically about the shank  23  and axially between the carburetor body  19  and the head  22  of the needle valve  17 . The spring-induced axial force produces increased frictional forces amongst the threads between the carburetor body  19  and the needle valve  17 , thus resisting needle valve rotation and alteration of the fuel flow setting. Unfortunately, springs  26  are relatively expensive to manufacture, and to produce sufficient frictional forces must be relatively long, causing the needle valves  17  to project a substantial distance outward from an otherwise compact carburetor. 
     One example of a stabilizing system for a fuel adjustment assembly is disclosed in U.S. Pat. No. 6,540,212, issued Apr. 1, 2003, assigned to Walbro Corporation, and incorporated herein by reference. This U.S. patent generally describes the carburetor fuel adjustment assembly  16  illustrated in  FIGS. 1 and 2 , having both the spring  26 , as described above, and a retainer or clip  27  which exerts a lateral force upon the spring  26  and indirectly upon the needle valve shank  23  to produce further friction and minimize unintentional valve rotation. Unfortunately, the retainer clip  27  is external to the carburetor body  19  and thus subject to possible damage. 
     Another example of a stabilizing system for a fuel adjustment assembly is disclosed in U.S. Pat. No. 5,948,325, issued Sep. 7, 1999, assigned to U.S.A. Zama, Incorporated, and incorporated herein by reference. In this U.S. patent, a resilient fastening member is press fitted into a pre-drilled bore of a carburetor body. Once located in the bore, two needle valve receptacle cavities are bored transversely into the body and completely through the fastening member, thus the fastening member has a diameter which is larger than the receptacle cavity. Threads are then formed in the receptacles by rolling threads into both the carburetor body and the fastening member, for threadable receipt of the needle valves. Because the fastening member is resilient, it does not undergo plastic deformation during thread rolling and does not form actual female threads as does the metal portion of the receptacle or carburetor body. When the needle valves are installed and adjusted, the fastening member produces a frictional force upon the male threaded valve shanks which assuredly holds the valves in their adjusted position. Unfortunately, manufacture of the fastening member is expensive because after installation into the carburetor body, it must be drilled to produce two axially spaced through-holes and the threads must be formed by the thread rolling. Moreover, the process of manufacturing the carburetor is restricted because both the receptacles and the fastening member must be machined simultaneously. Yet further, the bore required to receive the fastening member is relatively long because the through-holes, and thus the receptacles, are spaced radically away from one-another and with respect to the longitude of the bore. This requires a large portion of the carburetor body to be dedicated for the bore and fastening members, and which might otherwise be utilized for other carburetor features, producing a relatively larger and less compact carburetor. 
     SUMMARY OF THE INVENTION 
     A carburetor fuel adjustment assembly includes one and preferably two needle valves threaded into respective receptacles of a carburetor body and engaged with a retainer of resilient material. Each receptacle defines an elongated cavity which intersects a fuel passage. Each needle valve has a shank which threadably engages its associated receptacle and a tip axially movable relative to an orifice or seat by rotation of the shank to control fuel flow. The resilient retainer body frictionally engages and laterally biases the needle valve into a steadfast position relative to the orifice or seat. This assures the adjusted or set fuel flow does not change by resisting unintended needle valve movement due to such factors as engine vibration and factory installation of limiter caps, and until an intentional and sufficient torque is applied to the needle valve to change its setting or adjusted position. 
     The resilient retainer may be in the form of a plug or sleeve and preferably, is inserted into a bore after the receptacles are machined. Preferably the bore extends between and is preferably transverse to a pair of receptacles and in part opens into the receptacles. In this way, a portion of the plug or sleeve is exposed in each receptacle with a convex or cylindrical shape. Because such exposures are preferably substantially equal, the torque required to rotate each of the needle valves are substantially the same for both needle valves. 
     Objects, features and advantages of this invention include a carburetor fuel flow adjustment assembly which prevents inadvertent alteration of fuel flow after calibration, after factory installation of limiter caps and/or during user operation, improves engine control, improves engine performance and useful life, provides a compact carburetor design, reduces manufacturing costs and assembly costs, is relatively simple design, robust, inexpensive, requires little to no maintenance and in service has a long useful life. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the invention will become apparent from the following detailed description of the preferred embodiment(s) and best mode, appended claims, and accompanying drawings in which: 
         FIG. 1  is a side view of a prior art carburetor; 
         FIG. 2  is an exploded perspective view of a prior art mixture adjustment assembly of the carburetor of  FIG. 1 ; 
         FIG. 3  is a side view of a carburetor having a fuel adjustment assembly of the present invention with limiter caps removed to show internal detail; 
         FIG. 4  is a cross section of the carburetor taken generally along line  4 — 4  of  FIG. 3 ; 
         FIG. 5  is a bottom view of the carburetor; 
         FIG. 6  is an enlarged partial and exploded cross section of the carburetor taken generally along line  6 — 6  of  FIG. 3 ; 
         FIG. 7  is an enlarged partial cross section of a portion of the fuel adjustment assembly within circle  7  of  FIG. 6 ; 
         FIG. 8  is a cross section of a fuel orifice and tip of a needle valve of the fuel adjustment assembly taken along line  8 — 8  of  FIG. 6 ; 
         FIG. 9  is a perspective view of a retainer plug of the fuel adjustment assembly; 
         FIG. 10  is an end view of a modified form of a retainer sleeve; 
         FIG. 11  is a cross section of the retainer sleeve taken along line  11 — 11  of  FIG. 10 ; and 
         FIG. 12  is a fragmentary view showing a retainer portion of the retainer plug in a carburetor body taken along line  12 — 12  of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring in more detail to the drawings,  FIGS. 3–6  illustrate a carburetor with an adjustable needle valve and retainer assembly  30 , embodying the present invention. The fuel adjustment or needle valve assembly  30  controls fuel flow in a carburetor  34  for a combustion engine which is typically a gasoline powered two or four stroke spark ignition internal combustion engine. The carburetor  34  has a fuel-and-air mixing passage  36  through a carburetor body  38  and individually adjustable low and high speed needle valves  44 ,  46  each received in an associated receptacle  40 ,  42  in the carburetor body. The valves are threadably received in separate associated cavities  52 ,  54  each of which communicates with a separate coaxial fuel orifice or seat  56 ,  57  each disposed in a separate fuel passage  58  which communicates with the fuel-and-air mixing passage  36  to deliver fuel to the mixing passage. In operation liquid fuel is supplied to each cavity  52 ,  54  upstream of its orifice  56 ,  57  from a fuel reservoir or fuel metering chamber  60  through a passage such as passage  58  which is shown only for valve  44  and its associated cavity  52  and orifice  56 . A similar fuel supply passage communicates with cavity  54  upstream of its orifice or seat  57 . As shown in  FIG. 6  each needle valve  44 ,  46  has a shank  66  with male threads  70  which in assembly are threadably received in mating complimentary female threads  72  in each cavity  52 ,  54 . Each valve  44 ,  46  has a reduced diameter and preferably tapered tip  62  at one end which in assembly is received in part in its associated orifice  56 ,  57  and at the other end a head  68  the slot  69  therein for receiving the blade of a screwdriver to rotate the valve. In use, fuel flow is adjusted by rotating each valve in one direction to advance its tip  62  toward or further into its associated orifice or seat,  56 ,  57  to reduce fuel flow through its cavity,  52 ,  54  to the mixing passage  36  and rotated in the opposite direction to retract or withdraw its tip from its associated orifice  56 ,  57  to increase fuel flow through its cavity to the mixing passage. 
     The low and high-speed needle valves  44 ,  46  each preferably have a supplemental compression spring  64  which provides resistance against unintentional rotation of the needle valves  44 ,  46 . The supplemental spring  64  generally concentrically encircles the shank  66  of the needle valve  44 ,  46  and is compressed axially between the radially enlarged head  68  of the needle valve  44 ,  46  and the carburetor body  38 . The axial force produced by the compression springs  64  provides resistance which restrains rotation of the needle valves  44 ,  46  by indirectly creating friction between the male and female threads  70 ,  72  of the shanks  66  and the receptacles  40 ,  42  within the cavities  52 ,  54 . In contrast, the resilient retainer  32  adds to this resistance by creating friction directly between itself and preferably the male threads  70  of the shank  66  and laterally urging the threads  70  into engagement with the female threads  72  in the carburetor body  34 . Without use of the retainer plug  32 , the size of the compression spring would be considerably larger to create the same frictional force. In many applications, elimination of the spring may be preferred. Preferably, a resilient annular seal  76  is fitted sealably between the needle  62  and the respective receptacles  40 ,  42  in a counter bore  78  of the cavity  52 ,  54 . 
     The retainer may be in the form of a sleeve of a resilient plastic material located in a bore  82  of the carburetor body  38  having a centerline  84  which is substantially transverse and preferably perpendicular to and centered between the rotation axis  48 ,  50  of the low and high-speed needle valves  44 ,  46  (shown in  FIG. 7 ). The low speed cavity  52  is generally spaced laterally away from the high speed cavity  54  by a first distance  86 . Because the centerline  84  of the bore  82  is substantially centered between the rotation axes  48 ,  50  and the diameter  88  of the bore  82  is greater than the first distance  86 , the cavities  52 ,  54  generally communicate laterally with one-another laterally through the bore  82 . The retainer  32  preferably fits snugly into the bore  82  generally through a bottom  90  of the carburetor  34  (shown in  FIG. 5 ). An exterior cylindrical surface  92  of the sleeve  32  has a generally continuous and cylindrical portion  94  which is in tight contact with the carburetor body  38  in the bore  82 , and two diametrically opposite and convex portions  96 ,  98  exposed in the corresponding cavities  52 ,  54 , as best shown in  FIGS. 4 ,  7  and  12 . 
     As best shown in  FIG. 7 , the first portion  96  generally extends into the first cavity  52  by a first radial distance  100  and the second portion  98  generally extends into the second cavity  54  by a second radial distance  102 . The summation of the radial distances  100 ,  102  and the first distance  86  is generally equal to the diameter  88  of the bore  82 . Preferably, the first radial distance  100  is substantially equal to the second radial distance  102  for placing a substantially equal lateral force on the respective low and high speed needle valves  44 ,  46  creating a substantially equal and consistent torque required to rotate or adjust the needle valves. 
     Empirical data has demonstrated that use of the retainer or sleeve  32  will reduce tip  62  wobble by many magnitudes compared to the annular seal  76  alone. Reducing tip  62  wobble or lateral shifting decreases the change in shape and flow area of an annular area  104  between the tip  62  and orifice  56  and thus decreases changes in the carburetor fuel flow characteristics (see  FIGS. 4 ,  6  and  8 ). Preferably, the retainer or sleeve  32  is axially spaced substantially away from the tip  62  and preferably the seal  76  and is near the head  68 . The retainer  32  can be manufactured as a one piece body with a cylinder shape from a resilient material such as nylon, plastic or rubber. In carburetor applications having only one needle valve, the retainer  32  can be either a solid plug or a hollow cylinder or sleeve. In applications that have two needle valves the retainer  32  is preferably a sleeve of resilient material. 
     As best shown in  FIGS. 6–7  and  10 – 11 , utilizing a tube or sleeve  32  rather than a solid plug is preferable when the same retainer  32  stabilizes two needle valves  44 ,  46  because it reduces the effect of machining tolerances on each needle valve. Given conventional tolerances or clearance between the receptacle  40 ,  42  and valve shanks  66 , the sleeve  32  as tested had 0.008 inches to 0.020 inches of potential interference with each of the valve shanks  66 . With a sleeve, each needle valve regardless of the machining tolerance (i.e. centering of centerline  84  between rotation axes  48 ,  50 ) only needs to overcome the sleeve wall thickness flexure or yield strength. For instance, utilizing a nylon sleeve having an outer diameter  106  of 0.125 inches and a wall thickness  108  of 0.020 inches and establishing the first radial distance  100  being the minimum 0.008 inch thread to sleeve interference and establishing the second radial distance  102  being the maximum 0.010 inch thread to sleeve interference, both needle valves  44 ,  46  are shown to require substantially equal torques to achieve rotation. 
     Customarily, the low and high-speed needle valves  44 ,  46  of the carburetor  34  are adjusted and set at the factory by the engine manufacturer after the carburetor body  38  is mounted to a running combustion engine, not shown. If the fuel-and-air mixture is too lean, the running engine may overheat causing warranty concerns. If the fuel-and-air mixture is too rich, government regulatory emission requirements may be exceeded or violated. Therefore, limiting adjustment capability by the end user of the engine of the low and high-speed needle vales  44 ,  46  within an acceptable range is desirable. The engagement of known limiter caps  74  to the heads  68  of the valves  44 ,  46  establishes the end user adjustment range for fuel flow within the carburetor (i.e. neither too rich nor too lean). The limiter caps  74  are typically press fitted over the heads  68  in the factory after the proper fuel flow settings are made. Without the retainer  32  and after factory adjustment by the engine manufacturer, the press fitting of the limiter caps  74  to the heads  68  of either one or both of the needle valves  44 ,  46  may move the needle valves  44 ,  46  axially and/or laterally, causing the factory setting and prescribed adjustment range of the needle valves to be altered or changed. 
     Empirical data has demonstrated that use of the retainer or sleeve  32  will reduce tip  62  wobble by many magnitudes compared to the annular seal  76  alone. Reducing tip  62  wobble or lateral shifting decreases the change in shape and flow area of an annular area  104  between the tip  62  and orifice  56  and thus decreases changes in the carburetor fuel flow characteristics (see  FIGS. 4 ,  6  and  8 ). 
     While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. For instance, the carburetor  34  may have only one needle valve. In this application the retainer  32  may be the solid embodiment as opposed to the sleeve for cost or other considerations. Yet further, it is conceivable that retainer  32  rather than bearing directly upon the male threads  70  of the shank  66 , as illustrated, may also bear directly upon a smooth portion of the shank  66  which does not carry male threads. It is also conceivable that in many applications use of the spring  64  will not be required because the retainer will provide sufficient lateral force and axial restraint on the needle valves  44 ,  46  to prevent unintentional rotation. It is not intended herein to mention all the possible equivalent forms or ramifications of the invention. It is understood that the terms used herein are merely descriptive, rather than limiting, and that various changes may be made without departing form the spirit or scope of the invention.