US3284063A

US3284063A - Carburetor

Info

Publication number: US3284063A
Application number: US298263A
Authority: US
Inventors: James T Bickhaus; Dwight M Gordon; Forrest W Cook
Original assignee: ACF Industries Inc
Current assignee: ACF Industries Inc
Priority date: 1963-07-29
Filing date: 1963-07-29
Publication date: 1966-11-08
Anticipated expiration: 1983-11-08

Description

3,284,063 CARBURETOR James T. Bickhaus, Granite City, 111., Dwight M. Gordon, Farmington, MICIL, and Forrest W. Cook, Webster Groves, Mo., assignors to ACE Industries, Incorporated, New York, N.Y., a corporation oi New Jersey Filed July 29, 1963, Ser. No. 298,263 Claims. (Cl. 261-50) This invention is directed to an air valve carburetor of the type having an air and fuel mixture conduit through the carburetor body. Within the mixture conduit is mounted a manually operated throttle valve for movement between an open and closed position. Upstream from the throttle valve within the mixture conduit is mounted an air valve structure, which in the device of this invention measures the flow of air through the carburetor to meter the fuel flowing to the engine.

In an air valve carburetor of this type, the air valve is biased into a closed position. During engine operation, the differential of air pressure on opposite sides of the air valve is used to open the air valve against the closing bias. Also, flow of air into the mixture conduit may exert a dynamic air pressure on the air valve if it has an unbalanced construction. This tends to open the air valve against the closing bias. The closing bias is normally applied by a spring connected to the air valve. The opening force, resulting from the air pressure difierential on the opposite sides of the air valve, is applied by the unbalanced construction of the air valve and by the use of an air motor connected to the air valve and responsive to subatmospheric air pressures in the mixture conduit downstream of the air valve. A carburetor of this type is shown and described in the co-pending application of Cook et al., Serial No. 276,472, filed April 29, 1963, of which this invention may be considered a modification.

In carburetors of the type mentioned above and described in the above cited co-pending application of Cook et al. the air valve has a double function in controlling the amount offuel drawn into the mixture conduit of the carburetor. One, the air valve retains a partial vacuum downstream, which is varied in accordance with the degree of opening of the air valve. This partial vacuum is utilized to draw fuel from the main nozzle of the carburetor. Two, the degree of opening of the air valve greatly affects the flow of fuel to the main nozzle, since the fuel metering structure is directly tied to the air valve and functions to change the flow of fuel to the mixture conduit in accordance with the degree of opening of the air valve.

The air valve, as a measure of the air flow through the carburetor, is a sensitive device and is designed to act quickly to changes in flow of air through the carburetor. Thus, movement of the throttle lever manually to increase or decrease air flow to the engine results in a quick followup of the air valve to a position determined by the amount of the air flow through the carburetor. Also, change in engine speed due to changes in load will change the flow of air going to the engine through the carburetor and will result in a rapid change in the position of the air valve. Because of the tendency of the air valve to react quickly to changes in air flow through the mixture conduit, the air valve will swing past the position it should assume and then tends to hunt the correct position by fluttering back and forth over the position, which it should take. This hunting action of the air valve is undesirable. Since fuel flow is determined by the partial vacuum downstream of the air valve resulting from the position of the air valve as well as the position of the metering rod in the jet, an oscillating or hunting action of the air valve tends to cause an irregular flow of fuel detrimental to 3,284,053 Patented Nov. 8, 1966 proper operation of the engine. A more extreme problem exists, however, when upon rapid acceleration of the engine by a sudden opening of the throttle, the air valve will quickly sense the resulting increase in air flow and will swing widely to an open position past the position it should take. This lowers the partial vacuum downstream of the air valve and insufficient fuel will be drawn to mix with the increased air flow for providing the proper mixture for rapid acceleration.

It is thus an object of this invention to provide an air valve carburetor of the type described in which novel means are provided to prevent an undue leaning of the airfuel mixture upon rapid acceleration of the engine.

It is a further object of this invention to provide a novel air valve carburetor of the type described which will not swing or oscillate past a position to which it is quickly moved by the sudden increase in air flow through the carburetor.

It is another object of this invention to provide a novel air valve carburetor in which the tendency of the air valve to hunt its correct position is reduced or eliminated.

It is a further object of this invention to provide a novel air valve carburetor of the type described in which an appropriately enriched mixture is provided upon quick acceleration of the engine.

The invention is one which eliminates the tendency of an air valve in a carburetor from oscillating or hunting the proper position upon a sudden increase in air flow through the carburetor and also which eliminates the tendency of the air valve to lean out the air fuel mixture during rapid acceleration of the engine. The invention is in the use of means to provide a lag in the operation of the air valve so that the air valve will follow the opening of the throttle less closely and will not respond as quickly to a sudden increase in air flow to the carburetor. The invention specifically is in the use of a valve structure which permits a greater flow of air to the air motor operating the air valve than flow of air from the air motor. The valve structure provides a novel means for limiting the air flow from the air motor but does not hinder the flow of air to the air motor as the air valve moves in a closing direction. This action of the valve structure dampens the response of the air motor to an increase in air flow to the carburetor.

FIGURE 1 is a schematic representation of a carburetor in accordance with the invention.

FIGURE 2 is a partial sectional view of a carburetor utilizing the invention and of the type disclosed in FIG- URE 1.

FIGURE 3 is a valve seat member in accordance with the invention.

The carburetor shown in the figures has a body portion 10, which is connected at a flanged end 12 to an intake manifold structure 13 of engine E. The other end of the carburetor body is formed with an air horn section 14, to which an air filter may be mounted and through which air passes into the carburetor. Passing through the carburetor body 10 from the air horn 14 is a mixture conduit 16 opening at one end into the intake manifold 13 at 18 and through which air from the air filter passes to the engine. Mounted in the manifold end of the mixture conduit 16 is a throttle valve 20, fixed to a throttle shaft 22 journals-d for rotational movement in appropriate bearing surfaces in the carburetor body 10. Fixed to one end of the throttle shaft 22 is a manually operable lever 24 for moving the throttle 2t) from a closed to an open position. In the closed position the throttle 20 is across the mixture passage 16, as indicated in FIGURE 2, to prevent the flow of air therethrough. The throttle may be moved from this closed position to the wide open position of FIG- URE 1.

Mounted upstream of the throttle and across the air horn section 14 of the carburetor is an air valve 26. As indicated in FIGURE 1, valve 26 closes the air horn passage and extends across the mixture passage 16. When in this closed position, the air valve prevents flow of air through the mixture passage 16. Air valve 26 is fixed eccentrically to a shaft 28 journalled in the carburetor body for rotational movement. Shaft 28 extends through the carburetor subsantially parallel to the throttle shaft 22.

A short lever arm 30 is fixed to and extends from the air valve shaft 28. Lever arm 30 is loosely connected to one end of an actuating rod 32, the other end of which is fixed to a backing plate 34 of a diaphragm assembly 36 of a servo motor 39. The diaphragm assembly consists of a flexible diaphragm 38 of rubber or appropriate material, which has its center fixed between the pair of plates 34 and 35. The attached end of operating rod 32 may be spun over to lock the plates tightly together with the diaphragm 38 inbetween. The peripheral edge of diaphragm 38 is sandwiched between the flanged rim surface 40 of a spring housing 42 and a matching rim surface 43 of the body 10. Rim surfaces 40 and 41 are tightly fastened together by any appropriate means as by machine screws, for example, extending through the housing flange surface 40 into the adjacent portion of the carburetor body 10. A spring 44 is mounted within the housing 42 with one end abutting the closed end of the housing and its other end in contact with the center of plate 35 of the diaphragm assembly. Spring 44 biases the diaphragm assembly 30 in a direction to close the air valve 26.

The interior of housing 42 forms a chamber 45 with diaphragm 38 and is sealed except for an air passage 47 connecting into a first branch passage 48 opening into the intake manifold opening 18 through a port 51. Another second branch 49 of passage 47 extends through a port 50 into the mixture conduit 16 between the closed position of throttle valve and the air valve 26.

Formed within the body 10 of the carburetor is fuel bowl 52 adjacent to the mixing conduit 16. A fuel inlet fitting 56 extends through the fuel bowl wall to provide fuel access to the bowl 52. The fuel fitting 56 is connected in any appropriate manner to an inlet fuel line 58 leading from a fuel pump 60 which sucks fuel from a fuel tank 62. The fuel inlet line 58, fuel pump 60 and fuel tank 62 are only schematically shown in FIGURE 1.

Fuel fitting 56 connects with a short passage having a valve seat which is controlled by a needle valve 63 operated from a float lever 64 pivoted within the fuel bowl and having a free end attached to a float 68. The float controlled inlet needle 63 operates in a well-known manner. As the fuel in bowl 54 reaches a predetermined level, the float will force the pointed end of needle 63 into the valve seat to prevent additional flow of fuel into the bowl.

In the bottom of each fuel bowl there is a fuel jet 70 having a calibrated passage therethrough connected to a fuel passage 72 leading to a fuel well 74 formed in the carburetor body in the wall of mixing conduit 16.

Extending across mixture conduit 16 is a tubular across member 80 formed with a main fuel passage 82 and an idle fuel passage 84. An apertured fuel tube 86 extends downwardly into the Well- 74. The upper end of tube 86 is opened and connects with the fuel passage 82. Coaxially mounted within the fuel tube 86 is an idle fuel tube 88, with its upper open end connecting with the fuel passa-ge 84. Fuel tube 88 is closed except for a small restriction at its lower end suspended within the fuel well 74. An air passage 90 of predetermined size forms an air bleed passage from the fuel bowl 52 leading into the upper portion of well 74 to provide air flow into the main fuel passages. Passage 79 extends from the air horn 14 to the idle fuel passage 84 to provide a controlled air bleed through restriction 81 to the idle system. Another air passage 91 extends from the upper end of the air horn 14 and connects the idle fuel passage 84 to an idle cham- :ber 87, which is connected through idle parts 92 and 93 to the mixture conduit 16. A restriction 94 in passage 91 controls the amount of air bled into the idle system through passage 91.

Within the fuel bowl 52 there is mounted a fuel metering rod 96 which is connected at its upper end to an arm of lever 30 fixed to the air valve shaft 28. The other end of metering rod 96 is shaped with variations in rod thickness, consisting of an intermediate portion 100 of optimum thickness and a minimum thickness portion 103. The connection of metering rod 96 to the air valve shaft 98 provides simultaneous operation of the metering rod with the air valve so that fuel flow through the carburetor may be proportional to the air flow as measured by air valve 26.

An arm of the throttle lever 24 carries a screw for contacting a fast idle cam 106 freely pivoted on a screw shaft 108 and in an eccentric manner so that gravity will bias the cam 106 in a counterclockwise direction. A second lever 110 is also freely mounted for rotation on screw shaft 108 and is connected by a link 112 to a control lever 114 fixed to a shaft 116 journaled for free rotation in a cup housing 118.

A third lever 138 is fixed at the other end of the shaft 116 for rotation therewith. Lever 138 is bifurcated at its free end to receive one end of a bimetallic thermostatic spring 142 fitted into the bifurcation 140 so as to move the lever 138 to rotate shaft 116. The other end of spring 142 is fixed to a stationary shaft 143 extending from the cup housing 118.

Fixed to an intermediate portion of shaft 116 for rotation therewith is a second lever 126 carrying at its free end a tapered metering valve 128. The upper end of valve 128 is pivotally mounted on the free end of lever 126 so as to pivot relative thereto. The tapered end of the metering valve 128 extends into a restriction 130 having a passage therethrough of predetermined size which leads into an air chamber 132 in the housing 118. Air chamber 132 is connected by an air passage 134 to the air passage 47.

The cup housing 118 is divided into a pair of chambers and 146 by an imperforate wall structure 144 extending transversely across the housing. This wall 144 prevents the interference of air flow through one chamber with the air flow conditions of the other chamber. Chamber 146 is connected to the atmosphere through a vent 147. Chamber 145 is connected to a source of heated air such as a stove connected to the exhaust manifold of the engine. Warm air from the stove is brought through an inlet fitting 148 into the chamber 145. Chamber 145 is connected by an air passage 149 to the carburetor flange 12 at a point 150 downstream of the throttle 20. Because of subatmospheric air pressure conditions in the manifold 18 during the engine operation, air will flow from the manifold stove through the hot air conduit and fitting 148 into the chamber 145 and from the chamber through passage 149 into the manifold. Also, atmospheric air will flow through vent 147 into chamber 145 and when the valve 128 is open from chamber 145 through passage 134 into passage 47.

In operation, the engine is cranked to operate the fuel pump and to force fuel into the inlet fitting 56 of the carburetor. With the float valve in a lowered position, fuel will flow into the fuel bowl until the float 68 is raised to a pre-determined position at which point the needle valve 62 is closed to prevent further flow of fuel into the fuel bowl. Fuel will flow from the fuel bowl by gravity through the metering jet 70 into fuel passage 72 and to a level in the fuel well 74 equal to the level of fuel in the fuel bowl.

When the engine is cold thermostatic spring 142 rotates shaft 116 clockwise as viewed in FIGURE 1 to place the high point of cam 106 in the path of throttle screw 105 to hold the throttle open for fast idle opera;

tion. Valve 128 is moved out of slot 130 to permit air to bleed into passage 47 whereby air valve 26 is closed slightly to provide an enriched mixture for cold operation. When heated by warm air flowing through chamber 145, coil 142 rotates counterclockwise and closes valve 130 to provide a leaner mixture for the engine and places a lower portion of cam 106 in the path of idle screw 165 for curb idle operation.

One side of diaphragm 36 of the air motor is exposed to the air pressure within chamber 45, which is connected to port 50 in mixture conduit 16 between air valve 26 and throttle valve 20. The other side of diaphragm 36 from which rod 32 extends is exposed substantially to atmospheric pressure which is that of air in the air horn 14 upstream of air valve 26.

Starting During cranking of the engine with the throttle open, the engine turns over to pump air through the carburetor. With the engine cold, spring 44 of the servo motor holds air valve 26 closed. Servo motor spring 44 also biases the metering rod 96 through lever 98 to its uppermost position, so that the small diameter starting portion 99 of the metering rod is respectively positioned within the metering jet 70 to enable a sufiicient amount of fuel to flow into the fuel nozzle bar 80. Sufiicient air to start the engine passes through an aperture in the air valve 26 and into the mixture passage 16. Fuel to start engine is drawn out of the fuel well 75 and the

nozzle bar passages

82 and 84 by sub-atmospheric pressure at the nozzle bar 80. The fuel passes into the mixture conduits through the main nozzle ports 83 and the idle ports 92 and 93.

Engine operation When the engine is running and the throttle is opened from its closed position, the air pressure at port 50 drops from atmospheric and the air pressure in motor chamber 45 is reduced until diaphragm 36 is pressed inwardly against the bias of spring 44. The air valve 26 then takes a position determined by servo motor 39, in which the difference in air force on the opposite sides of diaphragm 36 balances the bias of spring 44. As throttle 20 is moved to change the amount of air flowing to the engine, the servo motor 39 will change the position of air valve 26 to retain substantially the same pressure drop across the air valve. The air valve 26 is thus a device for measuring air fiow through the carburetor to the engine.

The value of spring 44 is pre-selected to retain a fixed pressure drop across the air valve at all times. This spring determines the proper relationship between the amount of air flowing through the carburetor and the angular position of the air valve, which positions the proper portions of metering rod within jet 70. The shaped metering portions of rod 96 are calibrated to give sufiicient fuel flow through jets 70 to provide maximum power at any speed of the engine at wide open throttle up to the full air capacity of the carburetor. The optimum air-fuel ratio is maintained with a full open air valve by the increased depression around the nozzle bars 80 due to increased manifold vacuum at higher speeds.

Thus, as described above, the air valve 26 of the carburetor is used to provide a sub-atmospheric pressure at all times in the region of the nozzle bar 80 so that fuel can be pulled from the main fuel system through the main fuel jets 83 to mix with the air flow through the carburetor. Also the air valve is utilized to appropriately position the metering rod 96 within the jet 70 so as to control the flow of fuel to the mixture conduit 16 during engine operation.

The air valve and servo-motor assemblies in a carburetor of the type described is sensitive in operation and the air valve 26 is moved very quickly in response to changes in air flow through the mixture conduit. It has been found that there are certain disadvantages which accrue from an air valve which responds so readily and very rapidly to changes in air flow. It has been found that, when the air flow through the carburetor is rapidly increased, the air valve 26 reacts quickly to swing open toward its appropriate position. However, the inertia of the assembly carries the air valve past this position and opens the air valve too much. This results in excessive pressure in the servo-motor chamber 43, which will swing the air valve back through its proper position again. Thus, the air valve, under conditions of a sudden increase of air flow through the carburetor, tends to oscillate about the position its should assume, and thus to hunt the correct position. Such a condition occurs during rapid acceleration, when the throttle 20 is manually opened from a part open' position to a wide open position. The sudden rush of air through the carburetor causes the air valve to swing open and to move past its proper position for wide open throttle. This lowers the vacuum condition in the region of the main fuel nozzles 83 so that less fuel is drawn from the fuel circuit than is required for rapid acceleration. This leaning out of the mixture during the acceleration operation causes the engine to stumble before it gains its acceleration speed. Furthermore the hunting action of the air valve causes the vacuum condition downstream of the air valve to fluctuate, which results in poor engine operation during acceleration.

vAnother condition of engine operation, in which the described fluttering operation of the air valve occurs, is when there is a sudden change in the load of the engine with a fixed throttle position. If for any reason the load causes the engine to speed up, such as a downshifting in the transmission of the car, the sudden increase in air flow to the mixture conduit will cause the air valve 26 to swing beyond its proper position and thus again lean out the mixture.

' Therefore, in accordance with the invention and to eliminate the tendency of the air valve to hunt its proper position upon a sudden increase of air flow through the carburetor, a valve structure is positioned in the air passage 47 leading from servo-motor chamber 45 to the mixture conduit. The valve structure consists of a seat which may be formed by a small metallic disc or washer 1 60, as shown in FIGURE 3, which is press-fitted into passage 47 as indicated in FIGURE 2. Washer or disc has a central aperture bounded by peripheral portions 162 formed on a common circle. Scalloped corners or equally spaced portions 164 are formed between the peripheral circular portions 162. A small ball valve 166 is positioned on the seat 160 and is gravity biased into the aperture at the center of the disc 160. The ball' closes the central portion of the aperture defined by the circular peripheral portions 162, retaining, however, the scalloped portions 164 open for the passage of air from the servo-motor chamber 45 into the mixture conduit 16 under conditions of sub-atmospheric pressures at the

ports

50 and 51. A wire retainer 108 is positioned above the ball valve 166 so as to prevent the passage of the ball into the chamber 45, if the carburetor is turned upside down.

The operation of the ball check valve in passage 47 is such that under the conditions described above in which upon rapid acceleration of the engine when the throttle 20 is suddenly moved to a wide open position, manifold vacuum operates downstream of the air valve 26 to cause a flow of air out of the servo-motor chamber 45 through port 50. This flow of air is restricted due to the seating of the ball 66 on the circular peripheral portions 162. The scalloped apertured portions 164 of the valve seat permit only a limited flow of air through. Thus, the response of the servo-motor to an increase in vacuum conditions at port 50 is delayed, and the air valve opens at a much slower rate. The retarding then, of the air flow through passage 47 dampens the movement of the air valve 26 and also prevents it from oscillating around its new position upon sudden increase of air flow through the carburetor.

In a similar manner, any change in the load on the engine, such as a sudden decrease in the load, will cause the pressure downstream of the air valve 26 to drop due to a sudden increase in engine speed before the air valve moves. This causes the servo-motor 39 to operate quickly to position the air valve, as a measure of the increased air flow. However, the flow of air out of the servo-motor chamber 45 past the ball valve 166 is restricted so that the movement of the air valve is retarded and it moves more slowly than it would otherwise, into the new position it takes as a measure of the increased flow of air.

The delay in the opening of the air valve under the demands of the engine in the manner described above, prevents a rapid drop in the vacuum conditions around the nozzle apertures 83 and retains for a longer period of time the higher vacuum conditions. This results in a larger amount of fuel flowing out of the nozzle apertures 83 than would flow if the air valve opening were not dampened. This is a distinct advantage during rapid acceleration when a richer mixture is desired in the in creased power demands of the engine. Thus, by eliminating the tendency to lean out the fuel mixture by an over-opening of the air valve, there is now no tendency of the engine to stumble or falter and thus to produce a bump effect during rapid acceleration or in down-shifting of the transmission from a higher to a lower gear ratio, for example.

The type of valve structure described and shown in FIGURES 2 and 3 is not limiting as it is quite obvious that other valves having by-pass bleed structures would also function in a similar manner. The use, thus, of a valve that will retard the flow of air to the servo-motor passage 47 in one direction but not in the other still retains the high sensitivity of the air valve and servomotor combination, which is required so that the air valve may rapidly follow the movement of the throttle 20 and yet, at the same time, removes the disadvantage provided by the over-travel of the air valve under the conditions described above.

We claim:

1. A carburetor comprising a body structure having an air and fuel mixture conduit therethrough, a throttle valve mounted across said mixture conduit for movement from an open position to a position closing said mixture conduit, means for operating said throttle valve, an air valve mounted within said mixture conduit anteriorly of said throttle valve for movement from an open to a position closing said mixture conduit, means for supplying fuel to said mixture conduit, a servo motor means connected to said air valve to move said air valve between said open and closed positions thereof, means forming an air passage from said mixture conduit to said servo motor to therethrough operative to pass air when said valve mem her is seated.

2. The invention of claim 1 wherein said servo motor includes a chamber and a movable member having one surface exposed to the atmosphere and an opposite surface enclosed within said chamber, said air passage being connected to said chamber to control the position of said air valve in accordance with the drop in pressure of air flowing through said mixture conduit across said air said fuel reservoir to said mixture conduit between said.

air valve and said throttle valve, a metering rod having a portion of varying thickness positioned in said fuel passage for varying fuel flow theret-hrou-gh, means connecting said metering rod to said air valve for simultaneous movement therewith, an air motor connected to said air valve and metering rod to move said air valve and metering rod simultaneously, means forming an air passage from said mixture conduit to said air motor to control said air valve and metering rod in response to air pressure drop across said air valve and said throttle valve, and automatic unitary valve means in said air passage having provision for relatively free flow of air to said air motor from said mixture conduit when in a first operative position and for limiting the flow of air from said air motor to said mixture conduit when in a second operative position.

4. The invention of claim 3 wherein said air motor includes a chamber and a movable member having one surface exposed within said chamber, said air passage being connected to said chamber to control the position of said air valve in accordance with the drop in pressure of air flowing through said mixture conduit across said air valve.

5. The invention of claim 3, wherein said valve means includes a valve seat and a valve member biased onto said valve seat, said valve seat having openings therethrough for a limited flow of air when said valve member is on said valve seat.

References Cited by the Examiner UNITED STATES PATENTS 2,969,783 1/1961 Braun. 2,988,345 6/1961 Kolbe et al 26150 2,996,051 8/1961 Mick 26169 X 3,053,240 9/ 1962 Mick 261-69 X 3,190,284 6/1965 Koepf 1375l3.5 X 3,190,623 6/1965 Ball 26139 FOREIGN PATENTS 515,040 10/ 1939 Great Britain.

HARRY B. THORNTON, Primary Examiner.

T. R. MILES, Assistant Examiner.

Claims

1. A CARBURETOR COMPRISING A BODY STRUCTURE HAVING AN AIR AND FUEL MIXTURE CONDUIT THERETHROUGH, A THROTTLE VALVE MOUNTED ACROSS SAID MIXTURE CONDUIT FOR MOVEMENT FROM AN OPEN POSITION TO A POSITION CLOSING SAID MIXTURE CONDUIT, MEANS FOR OPERATING SAID THROTTLE VALVE, AN AIR VALVE MOUNTED WITHIN SAID MIXTURE CONDUIT ANTERIORLY OF SAID THROTTLE VALVE FOR MOVEMENT FROM AN OPEN TO A POSITION CLOSING SAID MIXTURE CONDUIT, MEANS FOR SUPPLYING FUEL TO SAID MIXTURE CONDUIT, A SERVO MOTOR MEANS CONNECTED TO SAID AIR VALVE TO MOVE SAID AIR VALVE BETWEEN SAID OPEN AND CLOSED POSITIONS THEREOF, MEANS FORMING AN AIR PASSAGE FROM SAID MIXTURE CONDUIT TO SAID SERVO MOTOR TO MOVE FROM AIR VALVE IN RESPONSE TO AIR PRESSURE DOWNSTREAM OF SAID AIR VALVE, AND VALVE MEANS IN SAID AIR PASSAGE FOR RESTRICTING THE FLOW OF AIR FROM SAID SERVO MOTOR TO SAID MIXTURE CONDUIT, SAID VALVE MEANS BEING CONSTRUCTED TO PERMIT RELATIVELY FREE FLOW FROM SAID MIXTURE CONDUIT TO SAID SERVO MOTOR, SAID VALVE MEANS COMPRISING A VALVE MEMBER AND A SEAT HAVING AT LEAST ONE OPENING THERETHROUGH OPERATIVE TO PASS AIR WHEN SAID VALVE MEMBER IS SEATED.