US3278171A

US3278171A - Carburetor

Info

Publication number: US3278171A
Application number: US319168A
Authority: US
Inventors: Harold A Carlson
Original assignee: ACF Industries Inc
Current assignee: ACF Industries Inc
Priority date: 1963-10-28
Filing date: 1963-10-28
Publication date: 1966-10-11
Anticipated expiration: 1983-10-11

Description

Oct. 11, 1966 H. A. CARLSON 3,278,171

CARBURETOR Filed Oct. 28, 1963 2 Sheets-Sheet 1 INVENTOR. HAROLD A. CARLSON Oct. 11, 1966 CARLSQN 3,278,171

CARBURETOR Filed Oct. 28, 1963 2 Sheets-Sheet 2 C 22 0 W "fii igfiinll INVENTOR. HAROLD A CARLSON United States Patent 0 Filed on. 2s, 1963, Ser. No. 319,168 4 Claims. or. 2e1 39 This invention is directed to an air valve carburetor for an internal combustion engine and i of the type having an air and fuel mixture conduit through the carburetor body. A throttle valve is mounted for movement between an open and closed position within the air and fuel mixture conduit. Upstream from the throttle valve within the mixture conduit is an air valve structure which measures the flow of air through the carburetor to meter the fuel flowing to the engine.

In an air valve carburetor of this type, such as disclosed in the copending application of Cook et al., Serial No. 276,472, filed April 29, 1963, the entire disclosure of which is incorporated by this reference, the air valve is biased to closed position. The opening of the air valve is determined by the air pressure differential on opposite sides of the air valve which differential is applied by an air motor connected to the air valve and responsive to subatmospheric pressures in the mixture conduit downstream of the air valve. The closing bias is partly applied by a temperature responsive spring connected to the air valve, and the spring expands or relaxes upon engine warm-up to decreased the closing bias. A metering rod for the fuel bowl of the carburetor is normally connected to the air valve and varies the flow of fuel into the fuel bowl in relation to the position of the air valve. For example, when the air valve is opened to its maximum position for high speed operation, the flow of fuel into the fuel bowl as determined by the metering rod is at a maximum.

In order that the engine may start firing during cranking, a rich mixture should be initially supplied. To assure a rich mixture, the carburetor must be choked; that is, its air supply must be cut off or diminished by the closing of the air valve. In addition, a relatively large amount of fuel should be supplied during cranking of the engine. A cold engine requires a rich mixture while a warm or hot engine requires a leaner mixture. Immediately upon engine starting, the engine should be supplied with a leaner mixture than that used in cranking as the mixture required for cranking is too rich. If the engine is cold or does not have a sufficiently high temperature to actuate the temperature responsive means, the intake manifold vacuum may be utilized and an important object of the invention is the use of such a vacuum to open the air valve to a predetermined amount immediately upon engine starting. This is possible because immediately upon engine starting, the intake vacuum increases to such a degree that it may be employed for this purpose.

An object of the present invention is'the provision of an air valve carburetor in which the air valve is opened by a secondary air motor to a predetermined minimum position immediately after engine starting with the air valve normally remaining at such position until the engine warms up. Thus, during engine cranking, the air valve provides a choking action but once the engine is started, the air valve is quickly snapped to a partly open position by the secondary air motor and permits a suflicient flow of air for optimum engine operation.

A further object of this invention is the provision of an air valve carburetor in which the flow of fuel is immediately decreased simultaneously with the snapping of the air valve to partly open position upon starting of the engine thereby to lean down the rich starting airfuel mixture. This is accomplished primarily by a second- 3,278,171 Patented Oct. 11, 1966 ary metering rod which is inoperable after engine warmup but is effective to control the flow of fuel during engine cranking and cold engine operation.

Briefly described, the present invention comprises an air valve carburetor in which a secondary air motor and a temperature responsive coil are operatively connected to the air valve shaft, the secondary air motor being responsive to subatmospheric air pressures in the mixture conduit downstream of the air valve to position the air valve against the closing bias of the temperature responsive coil upon c-old engine starting. A secondary fuel metering rod is operatively connected to the secondary air motor and upon engine starting, the associated fuel metering rod is simultaneously actuated with the secondary air motor to decrease the flow of fuel for providing an optimum air-fuel ratio immediately upon engine starting. The temperature responsive coil and a primary air motor take over the operation or positioning of the air valve once the engine warms up and the secondary air motor and associated fuel metering rod no longer control the air valve and air-fuel mixture. By having an auxiliary metering rod effective during engine starting and cold engine operation, fuel is readily supplied to a fuel well when the demand is high while permitting a primary metering rod to control effectively the flow of fuel after engine warm-up.

The invention accordingly comprises the constructions hereinafter described, the scope of the invention being indicated in the following claims.

In the accompanying drawings, in which one of various possible embodiments of the invention is illustrated,

FIGURE 1 is a schematic view of an air valve carburetor in accordance with this invention and showing the operation of the carburetor during cranking of the engine in cold weather;

FIGURE 2 is a schematic view of the carburetor illustrated in FIGURE 1 but showing the operation of the carburetor under normal cold running conditions of the engine; and

FIGURE 3 is a fragment of FIGURE 2 showing the operation of the secondary air motor and associated metering rod under normal warm engine operation.

Corresponding reference characters indicate corrres ponding parts throughout the several views of the drawings.

Referring to the drawings, a carburetor has a body 10 forming an air and fuel conduit 12. A lower flanged end 14 of body 10 is connected to an intake manifold 16 of an engine E. Mounted on an upper air horn section 18 of body 10 is an air filter 20 through which air passes into air and fuel mixture conduit 12.

Mounted in the manifold end of air and fuel mixture conduit 12 is a throttle valve 22 fixed to a throttle shaft 24 journaled for rotational movement on bearing surfaces in body It). In a closed position as shown in FIG- URE 1, throttle valve 20 is across air and fuel mixture conduit 12 and may be moved manually to the open position shown in FIGURE 2 under normal operating conditions.

Mounted upstream of throttle valve 22 and across air horn section 18 is an air valve 26. During the cranking of the engine, air valve 26 extends across air and fuel mixture conduit 12 as shown in FIGURE 1, and is biased to close the air horn passage. Air valve 26 is fixed eccentrically to a shaft 28 journaled in carburetor body 10 for rotational movement. Shaft 28 extends through the carburetor generally parallel to throttle shaft 24. A short lever arm 30 is fixed to and extends from air valve shaft 28. Lever arm 36 is loosely connected to one end of an actuating rod 32. Mounted on the opposite end of actuating rod 32 is a diaphragm assembly 34 of a primary servo or air motor indicated generally at 36. A spring 38 is mounted within a housing 40 of air motor 36 and biases diaphragm assembly 34 in a direction to close air valve 26.

Diaphragm assembly 34 forms with housing 40 a chamber 42 sealed except for an air passage 44 which communicates with air passage 46 extending through carburetor body 10. Air passage 46 communicates with a branch passage 48. Port 50 in air passage 48 opens into mixture conduit 12 upstream of throttle valve 22. Port 52 in air passage 46 opens into mixture conduit 12 downstream of throttle valve 22.

Restrictions

54 and 56 may be placed in air passages 46 and 48, respectively, and are formed of a predetermined size for determining the air pressure exerted through passages 46 and 48.

Formed within body 10 of the carburetor is a fuel bowl 60. A fuel inlet passage is shown diagrammatically at 62 and is connected to an inlet fuel line 64, which leads from a fuel pump 66. Fuel is delievered from a fuel tank 68 by fuel pump 66 through fuel passage 64 to fuel bowl 60.

A needle valve (not shown) in inlet passage 62 is operatively connected to a float 70 and operates in a well-known manner. In the bottom of fuel bowl 60 a fuel jet 72 has a calibrated passage therethrough connected to and leading to a fuel well 74. Mounted in the upper portion of fuel well 74 are inner and outer concentric fuel tubes 75 and 77 respectively. A tubular cross member 78 has a main fuel passage 80 and an idle fuel passage 82. Nozzle ports 84 through the wall of main fuel passage 84 allow fuel to pass to the mixture conduit 12. The upper end of inner fuel tube 75 communicates with idle passage 82 while the upper end of outer fuel tube 77 communicates with fuel passage 80.

Air passages 88 lead to idle fuel passage 82 from mixture conduit 12 upstream of air valve 26. Idle port 92 communicates with idle chamber 94 upstream of throttle valve 22 when in closed position. Idle port 96 communicates with idle chamber 94 downstream of throttle valve 22. Idle air and fuel flow through port 96 is controlled by an idle adjustment screw 98.

An accelerating pump piston 100 is mounted in a pump cylinder 102 and is linked to throttle 22 to provide additional fuel during opening of throttle valve 22. When throttle valve 22 is open, piston 100 forces fuel through an accelerating fuel passage to accelerating nozzle 104 extending into mixture conduit 12. A main fuel metering rod 106 is connected at its upper end to lever 30. The lower end of metering rod 106 has variations in rod thickness to vary the amount of fuel fed into fuel bowl 60 in relation to the position of diaphragm assembly 34.

An air passage 108 has an inlet port 110 at one end through body 10 upstream of air valve 26. An outlet port 112 extends through body 10 at a lower end of air passage 108. Adjustment screw 114 controls the flow of air bypassing air valve 26 and throttle valve 22. During engine idling with air valve 26 substantially closed, sufficient air for a proper air-fuel ratio is obtained through air passage 108.

A lever 118 is fixed to air valve shaft 28 for rotation therewith. Lever 118 is bifurcated at its free end to receive one end of a bimetallic thermostatic coil spring 120. The end of spring 120 is fitted into the bifurcated portion of lever 118 in such manner so as to rotate lever 118 in either direction. The other end of spring 120 is fixed to a stationary shaft 122 mounted on a cup housing 124 in which spring 120 is mounted.

Cup housing 124 forms a chamber 126 connected to the exhaust manifold of the engine by inlet 128 extending through housing 124. A suitable hot air conduit extends from the exhaust manifold to inlet 128. A return air passage 130 leads from chamber 126 to the manifold. Because of subatmospheric air pressure conditions in manifold 16 during engine operation, air flows from a manifold stove through the hot air conduit to inlet 128 into chamber 126, and from chamber 126 through passage 130 back into the manifold. For further details, reference may be made to aforementioned copending patent application Serial Number 276,472.

An important feature of this invention is the provision of a secondary air motor indicated generally at 132. Air motor 132 comprises a cylinder 134 in which a piston 136 is mounted for back and forth movement. An air passage 137 extends from cylinder 134 to a port 138 in mixture conduit 12 downstream of throttle valve 22. An actuating rod 139 is secured at one end to piston 136 and movable therewith. Pivotally mounted on the opposite end of rod 139 is a connecting link 140, having a laterally projecting lug 142. Mounted on shaft 28 for relative free rotation is a lever 144 having an arcuate slot 146 receiving lug 142. A projecting lug 148 on lever 144 is adapted to engage a finger 150 on lever 152 fixed to shaft 28. Mounted on one side of lever 152 for movement therewith is an auxiliary metering rod 154 which is operable only during engine cranking and cold engine operation and has a tapered lower end 156. Receiving tapered end 156 is a fuel jet 158 leading to fuel well 74. As shown in FIGURE 3, tapered end 156 is seated within fuel jet 158 and prevents flow of fuel therethrough during warm engine operation. Thus, main metering rod 106 controls the flow of fuel to fuel well 74 during Warm engine operation.

Pivotally mounted on the other side of lever 152 is one end of a link 160. The opposite lower end of link 160 is pivotally connected to a lever 162 freely pivoted on a screw shaft 164. A projecting lug 165 on lever 162 engages a fast idle cam 166 freely mounted on screw shaft 164 in an eccentric manner with gravity biasing cam 166 in a counterclockwise direction as viewed in FIGURES 1 and 2. A lever 170 fixed to throttle shaft 24 carries an adjustable screw 172 for contacting fast idle cam 166, as indicated in FIGURE 1.

While air motor 132 is illustrated in the form of a vacuum actuated piston, it is to be understood that air motor 132 could be arranged in the form of a diaphragm actuated motor such as in motor 36.

In operation, the engine is cranked to operate fuel pump 66 and to force fuel into inlet 62 of the carburetor. With float valve 70 in a lowered position, fuel will flow into fuel bowl 60 until float 70 is raised to a predetermined position at which inlet 62 is closed to prevent further flow of fuel into the fuel bowl. Fuel flows from fuel bowl 60 by gravity through

fuel jets

72 and 158 into fuel lwell 74 to a level equal to the level of fuel in the fuel bow One side of diaphragm assembly 34 of air motor 36 is exposed to air pressure within chamber 42, which is connected to

ports

50 and 52 in mixture conduit 12. The other side of diaphragm assembly 34 is exposed to atmospheric pressure, which is substantially that of air in air horn 18 upstream of air valve 26. Likewise, one end of cylinder 134 is exposed to air pressure in mixture conduit 12 downstream of throttle valve 22, through port 138 and air passage 137. The other end of cylinder 134 is exposed to atmospheric pressure which is substantially that of air in air horn 18.

Starting With the engine cold, thermostatic coil 120 and spring 38 bias air valve 26 toward a closed position with a predetermined torque. During cranking of the engine with the throttle open, unbalanced air valve 26 opens sulficiently to let air into the manifold upon turning over of the engine with main metering 106 and secondary or auxiliary metering rod 154 being in the position shown in FIGURE 1. Fuel flow through fuel jet 158 is at a maximum during cranking. Fuel to start the engine is drawn out of fuel well 74 through nozzle ports 84 and

idle ports

92, 96, by the low pressure conditions downstream of closed air valve 26 during cranking. Metering rod 106 which is controlled by primary air motor 36, is in its lowermost position with the large diameter portion thereof across fuel jet 72. Metering rod 154 controlled by secondary air motor 132 and coil 120 is in raised position during cranking with its smallest diameter portion across fuel jet 158. Piston 136 remains in the position shown in FIGURE 1 during cranking as there is an insufiicient pressure drop downstream of throttle valve 22 to actuate piston 136. Likewise, diaphragm assembly 34 remains in its position shown in FIGURE 1 during cranking.

Cold engine operation Immediately upon starting, pressure drop downstream of throttle valve 22 sensed through port 138 and air passage 137 actuates piston 136 and moves it to the position shown in FIGURE 2. The movement of piston 136 rotates lever 144 and lug 148 in a counterclockwise direction. Engagement of finger 150 by lug 148 effects the rotation of lever 152 and air valve shaft 28 to open air valve 26 to the solid line position shown in FIGURE 2 permitting a flow of air past air valve 26 sufficient to provide an optimum air-fuel ratio immediately upon engine starting. Further, tapered end 156 of metering rod 154 is lowered within fuel jet 158 immediately upon engine starting as shown in FIGURE 2, thereby to effect an immediate -decre-ase in fiow of fuel into fuel well 74 from jet 158. Air motor 36 is not as responsive initially to subatmospheric pressures downstream of air valve 26 as is air motor 132 and metering rod 106 is not immediately snapped to the position shown in FIGURE 2. The movement of metering rod 106 upon engine starting is more gradual than the movement of auxiliary metering rod 154. As shown in FIGURE 2, lug 148 is in engagement with finger 150 during cold engine operation and shaft 28 is rotated against the bias of thermostatic coil 120.

As the engine warms up, thermostatic coil 120 relaxes and effects rotation of shaft 28 to the dotted line position of air valve 26 shown in FIGURE 2. Lever 152 rotates relative to lever 144 to space finger 150 from lug 148, as shown in FIGURE 3, during warm engine operation as thermostatic coil 120 biases air valve 26 to an open position. Metering rod 154 has its lower end 156 seated across fuel jet 158 to stop the How of fuel therethrough upon normal warm engine operation. Air motor 36 and metering rod 106 control the flow of fuel to fuel well 74 after engine warm-up. Piston 136 is calibrated to be actuated at a predetermined minimum vacuum depending on engine requirements and so long as the minimum vacuum is sensed through port 138, piston 136 remains in the position shown in FIGURE 2. Upon throttle valve 22 being opened to wide-open position, the vacuum will be greatly decreased and piston 136 will return to its position of FIGURE 1. This will result in additional fuel supplied to fuel well 74 under such conditions. The return movement of piston 136 to the position of FIGURE 1 will be gradual as the vacuum will decrease gradually upon movement of throttle valve 22 to wide open position.

Immediately upon starting of the engine, piston 136 is moved or snapped to its position shown in FIGURE 2 to open partly air valve 26 and to decrease flow of fuel to fuel well 74. Piston 136 and lug 148 remain in the position shown in FIGURE 2 as long as the predetermined minimum vacuum is maintained downstream of throttle valve 22. The minimum vacuum will vary according to engine requirements. After the engine is started, negative air pressure downstream of air valve 26 are sensed through

ports

50 and 52 to air motor 36 which will operate to open air valve 26 and raise metering rod 106 gradually.

During cold engine operation, because of the subatmospheric pressure in manifold inlet 16, there is a continuous flow of air from the exhaust manifold stove through inlet 128, chamber 126, and passage 130 back to manifold inlet 16. When the engine begins to run, the

air in the exhaust manifold stove becomes heated and the flow of air into chamber 126 becomes warm. The heated air in chamber 126 warms the thermostatic coil and causes it to relax and to move lever 118 in a counterclockwise direction from its position of FIGURE 1. This rotates shaft 28 and lever 152 to cause opening of air valve 26 past the solid line indication in FIGURE 2. The heating of coil 120 spaces finger from lug 148 as shown in FIGURE 3, and opens air valve 26 to the broken line position indicated in FIGURE 2. After thermostatic coil 120 is heated, the operation of piston 136' has no effect on air valve 26 as air motor 36 and coil 120 then control the position of air valve 26. Therefore, piston 136 and air motor 132 affect engine operation only during starting and cold engine operation. Once the engine is stopped, piston 136 returns to its position shown in FIGURE 1.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A carburetor comprising a body 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 c-onduit, means for operating said throttle valve, an air valve mounted within said mixture conduit upstream of said throttle valve for movement from an open position to a position closing said mixture conduit, means for supplying fuel to said mixture conduit, a thermostatic cOil operatively connected to said air valve and adapted to move said air valve toward an open position upon engine warmup, and air motor operatively connected to said thermostatic coil and said air valve and responsive to air pressure downstream of said throttle valve to move said air valve immediately upon engine starting to a partly open position thereby to vary the air and fuel mixture, said carburetor body including a fuel reservoir and a fuel passage extending from said fuel reservoir to said mixture conduit between the air valve and the throttle valve, a metering rod adapted to be positioned in said fuel passage for varying the fuel flow thereto, means operatively connecting the metering rod to said air motor and decreasing the flow of fuel to the mixture conduit when said air motor moves the air valve to partly open position upon engine starting, a second air motor operatively connected to the air valve and responsive to the air pres sure downstream of said air valve to move said air valve toward an open position, a second metering rod posi tioned in said fuel passage for controlling the flow of fuel thereto, means operatively connecting said second metering rod to said second air motor for increasing the flow of fuel to said fuel passage upon a pressure drop across said throttle valve, and means forming an air passage from the mixture conduit downstream of the air valve to said second air motor to actuate said sec-0nd air motor and said second metering rod in response to a pressure drop across said throttle valve 2. A carburetor comprising a body 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, an air valve shaft journaled for rotation in said body and extending transversely across said mixture conduit upstream of said throttle valve, an unbalanced air valve fixed to said air valve shaft within said mixture conduit for rotative movement therewith in response to air flow from a position closing said mixture conduit to an open position, a temperature responsive coil operatively connected to said air valve shaft and biasing said air valve to closed position under cold engine operation, a first lever fixed to said air valve shaft for rotational movement therewith, a second lever mounted on said air valve shaft for free relative rotation and adapted to move said first lever when rotated in one direction, an air motor operatively connected to said second lever for rotating said second lever and said first lever immediately upon cold engine starting against the bias of said temperature responsive coil, means forming an air passage from the mixture conduit downstream of the throttle valve to said air motor to move said air valve upon engine starting and warm-up to a partly open position, a fuel reservoir and a fuel passage extending from said fuel reservoir to said mixture conduit between the air valve and the throttle valve, and a metering rod adapted to be positioned in said fuel passage for varying the fuel flow thereto, said metering rod being connected to said first lever and decreasing the fiow of fuel to the mixture conduit when the air motor is actuated upon engine starting and said air valve is moved to partly open position.

3. A carburetor comprising a body 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 upstream of said throttle valve for movement from an open position to a position closing said mixture conduit, a primary air motor operatively connected to said air valve for controlling the movement of the air valve, a secondary air motor operatively connected to said air valve and responsive to air pressure downstream of the throttle valve to snap said air valve to a partly open position immediately upon engine starting thereby to vary the air and fuel mixture, a fuel well and a fuel jet permitting flow of fuel to the fuel well, a thermostatic coil operatively connected to said air valve and adapted to move said air valve toward an open position upon engine warm-up, and a metering rod operatively connected to said thermostatic coil and adapted to be positioned in said fuel jet for varying the fuel flow therethrough and including means stopping flow of fuel through said fuel jet to said fuel well upon engine warmup and permitting flow of fuel therethrough upon engine cranking and cold engine ope-ration.

4. A carburetor as set forth in claim 3 wherein said secondary air motor is operatively connected to said metering rod and moves said metering rod to a position for decreasing the flow of fuel to the fuel well immediately upon engine starting and simultaneous with the movement of the air valve to said partly open position.

References Cited by the Examiner UNITED STATES PATENTS 2,598,450 5/1952 Shalf 261- 2,942,596 6/1960 Carlson 26l--39 2,943,848 7/1960 Gordon et a1. 26139 2,996,051 8/1961 Mick 26 139 3,029,800 4/1962 Armstrong 261-50 3,053,240 9/1962 Mick 261-50 3,078,079 2/1963 Mick 26150 3,171,868 3/1965 Hamilton 26l39 3,190,622 6/1965 Sarto 26139 FOREIGN PATENTS 718,381 ll/ 1954 Great Britain.

HARRY B. THORNTON, Primary Examiner.

T. R. MILES, Assistant Examiner.

Claims

1. A CARBURETOR COMPRISING A BODY 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 UPSTREAM OF SAID THROTTLE VALVE FOR MOVEMENT FROM AN OPEN POSITION TO A POSITION CLOSING SAID MIXTURE CONDUIT, MEANS FOR SUPPLYING FUEL TO SAID MIXTURE CONDUIT, A THERMOSTATIC COIL OPERATIVELY CONNECTED TO SAID AIR VALVE AND ADAPTED TO MOVE SAID AIR VALVE TOWARD AN OPEN POSITION UPON ENGINE WARMUP, AND AIR MOTOR OPERATIVELY CONNECTED TO SAID THERMOSTATIC COIL AND SAID AIR VALVE AND RESPONSIVE TO AIR PRESSURE DOWNSTREAM OF SAID THROTTLE VALVE TO MOVE SAID AIR VALVE IMMEDIATELY UPON ENGINE STARTING TO A PARTLY OPEN POSITION THEREBY TO VARY THE AIR AND FUEL MIXTURE, SAID CARBURETOR BODY INCLUDING A FUEL RESERVOIR AND A FUEL PASSAGE EXTENDING FROM SAID FUEL RESERVOIR TO SAID MIXTURE CONDUIT BETWEEN THE AIR VALVE AND THE THROTTLE VALVE, A METERING ROD ADAPTED TO BE POSITIONED IN SAID FUEL PASSAGE FOR VARYING THE FUEL FLOW THERETO, MEANS OPERATIVELY CONNECTING THE METERING ROD TO SAID AIR MOTOR AND DECREASING THE FLOW OF FUEL TO THE MIXTURE CONDUIT WHEN SAID AIR MOTOR MOVES THE AIR VALVE TO PARTLY OPEN POSITION UPON ENGINE STARTING, A SECOND AIR MOTOR OPERATIVELY CONNECTED TO THE AIR VALVE AND RESPONSIVE TO THE AIR PRESSURE DOWNSTREAM OF SAID AIR VALVE TO MOVE SAID AIR VALVE TOWARD AN OPEN POSITION, A SECOND METERING ROD POSITIONED IN SAID FUEL PASSAGE FOR CONTROLLING THE FLOW OF FUEL THERETO, MEANS OPERATIVELY CONNECTING SAID SECOND METERING ROD TO SAID SECOND AIR MOTOR FOR INCREASING THE FLOW OF FUEL TO SAID FUEL PASSAGE UPON A PRESSURE DROP ACROSS SAID THROTTLE VALVE, AND MEANS FORMING AN AIR PASSAGE FROM THE MIXTURE CONDUIT DOWNSTREAM OF THE AIR VALVE TO SAID SECOND AIR MOTOR TO ACTUATE SAID SECOND AIR MOTOR AND SAID SECOND METERING ROD IN RESPONSE TO A PRESSURE DROP ACROSS SAID THROTTLE VALVE.