Abstract:
A charge forming apparatus for delivering a controlled mixture of fuel-and-air to an engine has a butterfly-type throttle-choke valve disposed in a fuel-and-air mixing passage. Incoming air flowing through a downstream primary venturi of the mixing passage creates a strong negative pressure which induces fuel flow through a feed passage into the venturi from a fuel metering chamber held at near atmospheric pressure. When the engine is decelerating, substantial closure of the choke valve creates a secondary venturi having a vacuum which dynamically counters the effect of the primary venturi vacuum reducing fuel flow through the feed passage and causing the engine to run leaner and preventing egine stalls and reducing emmissions.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a charge forming apparatus or carburetor, and more particularly to a charge forming apparatus having a throttling choke valve. 
   BACKGROUND OF THE INVENTION 
   Conventional carburetors for internal fuel combustion engines are known to have a fuel-and-air mixing passage for delivering a controlled ratio of fuel-and-air to the combustion chamber of a running two or four cycle engine. The mixing passage is defined by a body of the carburetor and has a venturi disposed between an upstream or air inlet region and a downstream or mixture outlet region of the passage. Generally controlling the amount of air flowing through the venturi is a butterfly-type choke valve disposed pivotally within the air inlet region of the mixing passage. During engine cold-start conditions, the choke valve is in a substantially closed position allowing only a small amount of air to flow through the mixing passage and thus creating the needed rich mixture of fuel-and-air for easy engine cold starts. Otherwise, during warm engine starts and warm running engine conditions, the choke valve is substantially open creating minimal air flow restriction. Generally controlling the flow rate of the fuel-and-air mixture flowing through an intake manifold to the combustion chamber of a running engine is a butterfly-type throttle valve, which is disposed within the mixture outlet region of the mixing passage. As the throttle valve rotates from a substantially closed position to a wide open throttle position, and with the choke valve in a substantially open position, the engine speed will increase from idle to maximum or full power. 
   Typically, a pressure differential measured between a substantially constant pressure fuel metering chamber of a metering assembly and the high vacuum venturi region of the mixing passage causes liquid fuel to flow from the fuel metering chamber and into the venturi region via a fuel feed passage and a fuel nozzle disposed at a radially inward portion of the venturi or venturi region of the mixing passage. As air flow increases through the venturi, dictated by the position of the throttle valve and the air demand of the combustion engine, the venturi vacuum increases thus causing the fuel flow through the fuel feed passage and nozzle to increase. In this way, an engine initially at idle speed will increase in rpm to wide open throttle conditions with the increasing flow rate of the fuel-and-air mixture. 
   The fuel metering chamber is held at near atmospheric conditions and near constant volume by a flexible diaphragm disposed directly between the metering chamber and a reference chamber. The metering chamber is defined between a bottom side of the body of the carburetor and a top surface of the diaphragm. The reference chamber is defined between a bottom surface of the diaphragm and a bottom cover of the carburetor which carries an opening or nozzle that vents the reference chamber to atmosphere and/or filtered air. An integral or remote fuel pump, commonly operated via pressure pulses usually from the crankcase of the two cycle engine or the intake manifold of a four cycle engine, supplies fuel to the metering chamber via a supply valve which opens and closes in response to movement of the fuel metering diaphragm. 
   Of course, many other structural and dynamic factors of the carburetor contribute toward an easy start and smooth running engine which are also required to meet government and regulatory emission requirements. For instance, linkages are known to exist between exterior levers of the choke and throttle valves which make the positions of each valve inter-dependent to a limited degree. Moreover, a plurality of idle and intermediate speed fuel orifices are known to be orientated in the mixture outlet region of the mixing passage on either side of the throttle valve when closed. Such low speed orifices typically communicate with a fuel chamber which receives a controlled amount of fuel from the fuel metering chamber via a supplemental fuel passage. Usually the supplemental fuel passage is restricted controllably by a threaded needle valve which when rotated enters or retracts from the passage thus adjusting the ratio of fuel-to-air in the mixture for stable running conditions at low engine speeds. 
   Depending upon the engine type, displacement, and application, carburetors can become very complex, having highly machined and detailed bodies which incorporate many more numerous moving parts than those described above. All of this adds to the weight, manufacturing cost and maintenance expense of the carburetor. Likewise, there exist some two cycle engine applications, such as that of small lawn and garden appliances where a more simplistic, lighter, and less expensive carburetor would be ideal. Unfortunately, known carburetors must generally include all the costly components described above to support an easy start and reliable running engine which also meet regulatory emission requirements. 
   SUMMARY OF THE INVENTION 
   A charge forming apparatus for delivering a controlled mixture of fuel-and-air to a combustion engine has a butterfly-type choke valve with throttling capability disposed in an air inlet region of a fuel-and-air mixing passage. Fuel is mixed with air in a venturi region of the mixing passage disposed between the air inlet region and a mixture outlet region of the mixing passage. A conventional throttle valve is not disposed in the mixture outlet region since the choke valve performs the throttling function. A strong negative pressure produced at the venturi region, or primary venturi, induces fuel flow through a fuel feed passage into the venturi region from a fuel metering chamber of a fuel metering system. The fuel metering chamber is held at near atmospheric pressure when the throttling choke valve is in a closed position for cold-engine starts or in an open position for running at high engine speeds. When the engine is decelerating and/or when the throttling choke valve is in an idle position, the fuel-and-air mixture ratio becomes leaner to prevent engine stalls and to reduce emissions. The fuel-and-air mixture is “leaned-out” by a secondary venturi disposed upstream of the primary venturi. The vacuum produced by the secondary venturi is substantially weaker than the vacuum produced by the primary venturi. However, the secondary venturi still has a dynamically countering effect to the primary venturi by reducing fuel flow through the fuel feed passage when the throttling choke valve is in the idle position. The secondary venturi is defined between an interior wall which defines the air inlet region and a plate of the throttling choke valve when in the idle position. The small clearance created between the plate and the interior wall produces the high air flow velocity which induces the vacuum exposed to a reference nozzle of a reference passage. The vacuum is transmitted via the reference passage to a reference chamber of the fuel metering system which is separated from the fuel metering chamber by a flexible diaphragm. When the throttling choke valve is not in the idle position, the plate pivots out of the position necessary to create the secondary venturi and the reference chamber is exposed to near atmospheric pressure. 
   Preferably, the carburetor body defines an air bypass channel which communicates directly between the air inlet region upstream of the choke valve and the mixture outlet region of the mixing passage. A threaded bypass screw or valve controllably restricts the bypass channel to fine tune the fuel-and-air mixture ratio to obtain stable engine running conditions at idling speed. Preferably, when the throttling choke valve is not in the idle position, a rich mixture of fuel-and-air is promoted via a vent passage and an isolation valve which communicates between a near atmospheric air source and the reference passage when the isolation valve is in the open state. 
   Objects, features, and advantage of this invention include a simplified carburetor which does not have a throttling valve and the associated linkages which would be required to mechanically interact with the choke valve. Moreover, engine stalls due to an overly rich mixture of fuel-and-air are eliminated at idle and deceleration operating conditions thereby providing a reliable smooth running engine with reduced emissions. Additional advantages are a reduced number of manufacturing parts, a design which is 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 of the preferred embodiment and best mode, appended claims, and accompanying drawings in which: 
       FIG. 1  is a cross-section of a charge forming apparatus of the present invention having a throttling choke valve shown in an idle position and in phantom a closed position; 
       FIG. 2  is the cross-section of the charge forming apparatus of  FIG. 1  with the throttling choke valve shown in a wide open position; 
       FIG. 3  is a cross-section of the charge forming apparatus having a solenoid-type isolation valve shown in a closed state when the throttling choke valve is in the idle position, and taken along line  3 — 3  of  FIG. 1 ; 
       FIG. 4  is the cross-section of the charge forming apparatus of  FIG. 3  with the solenoid-type isolation valve shown in an open state when the throttling choke valve is in the open position; 
       FIG. 5  is a cross-section of a modification of the charge forming apparatus of  FIG. 1  illustrating an isolation valve integral to the throttling choke valve and shown in the open state when the throttling choke valve is in a closed position; 
       FIG. 6  is the cross-section of the charge forming apparatus of  FIG. 5  illustrating the isolation valve in the closed state when the throttling choke valve is in the idle position; and 
       FIG. 7  is the cross-section of the charge forming apparatus of  FIG. 5  illustrating the isolation valve in an open state when the throttling choke valve is in the open position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring in more detail to the drawings,  FIGS. 1 and 2  illustrate a charge forming apparatus or carburetor  10  according to the present invention having a butterfly-type throttling choke valve  12 , and not having a conventional throttle valve. The throttling choke valve  12  is mounted within an upstream or air inlet region  14  of a fuel-and-air mixing passage  16  defined by and extending through a main body  18  of the carburetor  10 . From a substantially atmospheric air source  20 , air flows into the air inlet region  14  of the mixing passage  16 , flows about and past the throttling choke valve  12 , and into a high vacuum venturi region  22  of the mixing passage  16  defined by a primary venturi  24  carried by the body  18 . At the venturi region  22 , the air mixes with fuel flowing out of a fuel orifice or nozzle  26  disposed at the venturi  24 . The mixture of fuel-and-air then flows through a downstream or mixture outlet region  28  of the mixing passage  16 , and through an intake manifold of a two or four cycle engine (not shown). 
   Fuel flows through the fuel nozzle  26  via a fuel feed passage  30  defined by the body  18  and extending between the fuel nozzle  26  and a fuel metering chamber  32  of a fuel metering system  34 . The fuel metering system  34  functions as a fuel regulator receiving pressurized liquid fuel from a conventional fuel pump (not shown) and supplying fuel, usually at a sub-atmospheric pressure from the fuel metering chamber  32 , to the mixing passage  16  via the fuel feed passage  30  and nozzle  26 . The fuel metering chamber  32  is defined by the carburetor body  18  and an upward or first side  36  of a flexible diaphragm  38  sealed along a peripheral edge  40  to the body  18 . A dry air reference chamber  44  is defined by a downward or opposite second side  46  of the fuel metering diaphragm  38  and a bottom cover  48 . The peripheral edge  40  of the diaphragm  38  is thus compressed and engaged sealably between the body  18  and the cover  48  which engages the body  18  via some form of conventional fasteners (not shown). 
   Referring to  FIG. 2 , when the engine is running at full load, typically described as wide-open-throttle, the charge forming apparatus  10  of the present invention operates much like a conventional carburetor. That is, the throttling choke valve  12  is in a full open position  50  permitting maximum air flow to the air intake manifold of the engine. The reference chamber  44  of the fuel metering system  34  is vented to the near atmospheric pressure of the air inlet region  14  via a reference passage or external conduit  52  which extends between a reference orifice  54  disposed at the air inlet region  14  of the mixing passage  16  substantially near the venturi  24  and a connector passage  56  carried by the cover  48 . Moreover, the fuel nozzle  26  is exposed to the relatively high vacuum of the primary venturi  24  causing liquid fuel to flow from the fuel metering chamber  32  into the venturi region  22 . This flow is induced by the pressure differential between the vacuum created in the venturi region  22  by the primary venturi  24  and the near atmospheric pressure within the fuel metering chamber  32  which is maintained by the reference chamber  44  provided the choke valve  12  remains. 
   Although not shown, a fuel supply valve is preferably actuated by the diaphragm  38  to supply a quantity of fuel, via the fuel pump, to the fuel metering chamber  32 . When the diaphragm  38  flexes sufficiently upward as a result of fuel flowing out through the fuel feed passage  30 , a mechanical linkage in contact with an approximate center of the diaphragm preferably pivots moving an obstructing head of the valve off of a valve seat preferably carried by the main body  18  allowing a quantity of fuel to flow through a supply passage from the fuel pump. As the diaphragm flexes downward the mechanical linkage moves the valve head back upon the seat to obstruct further fuel from flowing into the metering chamber  32 . 
   As best shown in  FIG. 1 , the high speed fuel-and-air mixture ratio can be adjusted via a threaded needle valve adjustment screw  58  engaged threadable to the body  18 . The screw  58  has a tip or head  60  which adjustably restricts the fuel feed passage  30  when rotated, thus controlling the liquid fuel flow rate entering the venturi region  22  via the fuel nozzle  26 . Referring to  FIG. 2 , the complexity of the carburetor  10  can be simplified and the manufacturing costs reduced by eliminating the high speed adjustment screw  58  where engine manufacturer performance specifications do not require high speed fuel ratio adjustments. 
   When the engine is running at less than wide-open-throttle, the charge forming apparatus  10  is unlike conventional carburetors because the single throttling choke valve  12  functions to control the fuel-and-air mixture flow and throttle the speed of the engine by pivoting along arrow  61  between a substantially closed or idle position  62  and the full open position  50  instead of the typical separate throttle valve of the conventional carburetor. 
   When the operator rotates the throttling choke valve  12  toward the full open position  50  (as best shown in  FIG. 2 ), the air flow rate through the venturi region  22  increases causing an increase in vacuum pressure. The increase in vacuum pressure increases the differential pressure across the fuel feed passage  30  which causes an increase in fuel flow rate through the fuel nozzle  26  of the fuel feed passage  30 . With the increase in fuel flow rate and the increase in air flow rate, the fuel-and-air mixture ratio remains substantially constant provided the pressure within the fuel metering chamber  32  remains substantially constant. Moreover, as fuel flows out of the fuel chamber  32 , the diaphragm  38  is free to flex upward, thus opening the fuel metering valve to supply make-up fuel to the fuel chamber  32  from the fuel pump. 
   Unlike conventional carburetors, and during deceleration of a combustion engine, a disc-like plate  64  of the throttling choke valve  12  rotates within the air inlet region  14  of the mixing passage  16  toward the idle position  62  and away from the wide open position  50 . When the throttling choke valve  12  is in the idle position  62 , the plate obstructs approximately eighty-five to ninety percent of the flow cross sectional area of the air inlet region  14 . During the pivoting action of the plate  64  and even when the throttling choke valve  12  reaches the idle position  62 , the reference nozzle  54  remains orientated downstream of the plate  64 . 
   Because closure of the throttling choke valve  12  dictates or leads the running speed of the engine, as the plate  64  rotates and the choke valve  12  moves toward the idle position  62 , a clearance  66  defined between a circumferential outer edge  68  of the plate  64  and a cylindrical wall  70  of the air inlet region  14  becomes increasingly smaller causing air flow velocity through the clearance  66  or flow rate through the reduced flow area to increase. This velocity has a venturi-like effect which exerts a relatively higher vacuum upon the reference nozzle  54 , thus acting as a countering secondary venturi  72  disposed immediately upstream of the main venturi  24 . As a result of the secondary venturi  72  vacuum, the pressure within the reference chamber  44  decreases from near or slightly sub-atmospheric to a higher vacuum pressure which decreases the pressure differential across the feed passage  30  to reduce the rate of fuel flow from the fuel metering chamber  44  into the venturi region  22  via the feed passage  30  and nozzle  26 , thus decreasing the quantity of the fuel-and-air mixture supplied to the intake manifold of the combustion engine via the mixing passage  16  during deceleration and idle conditions. This vacuum pull transmitted via the reference passage  44  prevents an overly rich mixture of fuel-and-air which would stall the engine during deceleration or at idle. To increase engine rpm from idle, the throttling choke valve  12  is opened toward the full open position  50 , which causes the vacuum at the reference nozzle  54  and, thus reference chamber  44  to decrease allowing the quantity of the fuel-and-air mixture to increase for higher rpm and/or greater engine load operating conditions to occur. 
   During cold start conditions of the engine, the throttling choke valve  12  is generally in a closed position  74 . When closed, preferably essentially all of the flow area of the air inlet region is obstructed by the plate  64  except for a bleed hole  76  extending through the plate  64  which allows a small amount of air to flow for engine starting. When closed, the clearance  66  is essentially eliminated and the vacuum effect of the secondary venturi  72  is prevented. However, the mixture outlet region  28 , the venturi region  22  and that portion of the air inlet region  14  of the mixing passage  16  disposed downstream of the closed plate  64  are still under a vacuum as a result of the negative pressure pulses transmitted via the intake manifold of the engine during cranking and starting. 
   This vacuum created by engine cranking, with the throttling choke valve  12  closed, is transmitted via the reference passage  52  into the reference chamber  44 , and has the negative or countering effect on the diaphragm  38 . This would tend to lean-out the fuel-and-air mixture at a time when a relatively rich mixture is needed for cold starts. However, the diameter or sizing of the fuel and reference nozzles  26 ,  54  compensates for the negative countering effect of the engine cranking vacuum upon the reference nozzle  54  and a sufficiently rich mixture of fuel-and-air is provided for cold starts. To provide this rich mixture, the cross sectional flow minimum area of the fuel nozzle  26  must be considerably larger than the minimum air flow cross sectional area of the reference nozzle  54 . As an example, for a fuel nozzle diameter of 0.71 mm, the reference nozzle diameter is substantially within the range of 0.46 mm to 0.51 mm, depending upon the carburetor  10  application and performance requirements. Preferably, the ratio of the fuel nozzle verse reference nozzle minimum cross sectional areas is in the range of 1.67:1 to 1.50:1. The diameter of the air bleed hole  76  is approximately 3.7 mm, however, the air flowing through it does not produce a venturi-effect upon the reference nozzle  54  and thus does not increase the negative pressure directly at the nozzle  54 . Moreover, a preferred ratio range of the surface area of the first side  36  of the diaphragm  38  to the minimum cross sectional flow area of the reference nozzle  54  is generally in the range of 2116:1 to 140:1. 
   Preferably, the idle speed of the engine is fine tuned or adjusted via an idle adjustment screw valve  78  which is adjusted threadably to partially restrict an air bypass channel  80  defined by the body  18  and extending between a bypass aperture  82  exposed to near atmospheric pressure upstream of the throttling choke valve  12  and a bypass orifice  84  opening into the mixture outlet region  28  of the fuel-and-air mixing passage  16 . The idle adjustment screw valve  78  is primarily used to adjust for engine-to-engine variations and is particularly advantageous to meet idle specification requirements of an engine manufacturer. With a fuel nozzle  26  diameter of 0.71 mm, the bypass orifice  84  minimum diameter is preferably approximately 3.7 mm. 
   Preferably, the throttling choke valve  12  is releasably held in the idle position  62  by an exterior detent lever (not shown) such as that disclosed in U.S. Pat. No. 6,561,496, to Gliniecki et. al., issued May 13, 2003, and incorporated herein by reference. 
   Referring to  FIGS. 3 and 4 , an additional feature of the present invention is illustrated having an isolation valve  86  of a solenoid-type used for partially diverting the reference passage  52  to the atmospheric air source  20  upstream of the throttling choke valve  12  when the isolation valve  86  is in an open state  87  and the throttling choke valve  12  is in the open position  50  or the closed position  74 , but not when the throttling choke valve  12  is in the idle position  62 . Exposing the atmospheric pressure source  20  directly to the reference passage  52  promotes a rich fuel-and-air mixture to flow to the intake manifold which is needed for cold starts and for higher than idle rpm running conditions. When the throttling choke valve  12  is in the idle position  62 , the isolation valve  86  is in a closed state  89  (as best shown in  FIG. 3 ), depriving the reference chamber  44  of an atmospheric air source in order to promote a lean fuel-and-air mixture. 
   The carburetor body  18  defines a bore or chamber  88  of the solenoid-type isolation valve  12  which interposes the reference passage  52  into a first and second leg  90 ,  92 . The first leg  90  communicates between the reference chamber  44  and the valve chamber  88  via the reference aperture  56  and a reference port  94  disposed at the valve chamber  88 . The second leg  92  communicates between the valve chamber  88  via a vacuum port  96  and the air inlet region  14  via the reference nozzle  54 . When isolation valve  86  is in the closed state  89 , an actuating member or valve head  98  seats sealably against an annular valve seat  100  carried by the body  18  and exposed within the valve chamber  88 . The reference and vacuum ports  94 ,  96  are orientated on a common side of the seat  100  (or portion of the valve chamber  88 ). Therefore, the first and second legs  90 ,  92  are in continuous communication regardless of whether the isolation valve  86  is open or closed, and the reference chamber  44  continuously communicates with at least the reference nozzle  54 . 
   When the throttling choke valve  12  moves out of the idle position  62 , the isolation valve  86  moves to the open state  87  ( FIG. 4 ) by moving the valve head  98  axially or linearly away from the seat  100 . When valve  86  is open, vent port  102  of a vent passage  104  defined by the body  18  and disposed on an opposite side of the seat  100  from the reference and vacuum ports  94 ,  96 , is in communication with ports  94 ,  96 . Thus, the valve chamber  88  and the reference chamber  44 , via the first leg  90  of the reference passage  52 , are in communication with the atmospheric air source  20  via the vent passage  104 . Because the vent port  102  is separated from the reference and vacuum ports  94 ,  96  by the valve seat  100 , when the actuating member  98  moves linearly to engage the seat  100  and thus moves into the closed state  89 , the vent port  102  is isolated from the reference and vacuum ports  94 ,  96 . Therefore, the reference passage  52  communicates only between the reference chamber  44  and the air inlet region  14  via the reference aperture  56  and reference orifice or nozzle  54 . When the head  98  of the solenoid-type isolation valve  86  is unseated, all of the ports communicate with each other via the valve chamber  88 , thus the vent passage  104  is in communication with the reference passage  52  for reducing the vacuum within the reference chamber  44  and effectively increasing the rate at which the fuel-and-air mixture flows into the intake manifold. 
   Preferably, the solenoid-type isolation valve  86  is de-energized when in the open state  87  and the throttling choke valve  12  is in the closed position  74  or the open position  50 , but not in the idle position  62 . In this way, the isolation valve  86  need not be energized when attempting to start the engine with a closed throttling choke valve  12 . 
     FIGS. 5–7  illustrate a modified mechanical isolation valve  86 ′ which replaces the solenoid actuated isolation valve  86  and is integrated with the rotating shaft  106 ′ of the throttling choke valve  12 ′. The shaft  106 ′ is rotatably seated within a bore  108  carried by the body  18 ′ and extends transversely through the air inlet region  14 ′. The plate  64 ′ of the throttling choke valve  12 ′ is engaged rigidly to the shaft  106 ′ and rotates or pivots with the shaft  106 ′ between the closed and open positions  74 ′,  50 ′. The valve chamber  88 ′ is part of the bore  108 ′ and is generally defined between the body  18 ′ and the shaft  106 ′ and is further isolated from the air inlet region  14 ′ by a tight radial fit or close tolerance between the body  18 ′ and a cylindrical portion of the shaft  106 ′ disposed between the air inlet region  14 ′ and the valve chamber  88 ′. 
   Like the solenoid-type isolation valve previously described, the reference port  94 ′, the vent port  102 ′ and the vacuum port  96 ′ of the integrated isolation valve  86 ′ are carried by the body  18 ′ and disposed at the valve chamber  88 ′. Moreover, the vacuum port  96 ′ is preferably disposed between, and spaced circumferentially apart from the reference and vent ports  94 ′,  102 ′. A circumferentially extending recess  110  of the elongated shaft  106 ′ is open radially outward and aligns axially to and is thus in communication with the valve chamber  88 ′ of the bore  108 . The recess  110  extends circumferentially approximately 340 degrees. The remaining twenty degrees, which has a circumferential surface which is flush with the outer cylindrical surface of the shaft  106 ′, provides the valve head  98 ′ of the rotating shaft  106 ′. The recess  110  is rotatably received in the bore  108  and sealed therewith adjacent its axial edges. 
   When the integrated throttling choke valve  12 ′ is in the closed position  74 ′ as best shown in  FIG. 5 , the recess  110  of the elongated shaft  106 ′ is misaligned circumferentially to the vacuum port  96 ′. The valve head  98 ′, which is aligned circumferentially to the vacuum port  96 ′, isolates the vacuum port  96 ′ and associated second leg  92 ′ of the reference passage  52 ′. The reference chamber  44 ′ is thus vented solely to the atmospheric air source  12 ′ via the vent passage  104 ′, the recess  110  and the first leg  90 ′ of the reference passage  52 ′. 
   When the integrated throttling choke valve  12 ′ is in the idle position  62 ′, as best shown in  FIG. 6 , the recess  110  is misaligned circumferentially to the vent port  102 ′. The valve head  98 ′, which is aligned circumferentially to the vent port  102 ′, isolates the vent port  102 ′ and associated vent passage  104 ′. The reference chamber  44  is thus exposed solely to the vacuum induced at the secondary venturi  72 ′ via the first and second legs  90 ′,  92 ′ and the intersecting recess  102 ′. 
   When the integrated throttling choke valve  12  ′ is in a position opened wider than the idle position  62 ′, or is in the open position  50 ′, as best shown in  FIG. 7 , the recess  110  is aligned circumferentially to the reference, vacuum and vent ports  94 ′,  96 ′,  102 ′. The valve head  98 ′ is misaligned circumferentially to all the ports within the valve chamber  88 ′. Consequently, the reference chamber  44 ′ is exposed to the atmospheric air source  20 ′ via the first leg  90 ′, the recess  110  and the vent passage  104 ′, and is exposed to the pressure at the reference nozzle  54 ′, which is near atmospheric pressure when the choke valve  12 ′ is in the open position  50 ′, via the second leg  92 ′, the recess  110  and the first leg  90 ′ of the reference passage  52 ′. 
   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 ramifications 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.