US3618904A

US3618904A - High velocity pressure diaphragm carburetor

Info

Publication number: US3618904A
Application number: US846563A
Authority: US
Inventors: Stephen Woods
Original assignee: Individual
Current assignee: RYCOMBEL ENTERPRISES Inc
Priority date: 1969-07-31
Filing date: 1969-07-31
Publication date: 1971-11-09
Anticipated expiration: 1988-11-09

Abstract

The disclosure relates to a carburetor having a closed fuel reservoir or chamber to eliminate vapor losses. A mechanical linkage including a cam operated by the throttle valve controls fuel flow to the reservoir or chamber at off-idle, part-throttle and wide-open throttle.

Description

United States Patent 14155 Shadywood Drive, Plymouth, Mich.

Inventor Stephen Woods Appl. No. 846,563

Filed July 31, 1969 Patented Nov. 9, 1971 Continuation-impart of application Ser. No. 822,828, Apr. 9, 1969, now Patent No. 3,556,067, which is a continuation-in-part of application Ser. No. 629,488, Apr. 10,

1967, now abandoned.

HIGH-VELOCITY PRESSURE DIAPHRAGM CARBURETOR 14 Claims, 3 Drawing Figs.

[52] U.S.Cl 26l/23A, 26l/39A,26l/69A,26l/5l [5i] lnt.Cl F02m 7/06 [50] FieldolSearch 26l/39.i, 69.l,5l,23.l

Primary Examiner-Tim R. Miles Attorney-Whittemore, Hulbert & Belknap HIGH-VELOCITY PRESSURE DIAPHRAGM CARBURETOR This application is a continuation-in-part of my application Ser. No. 822,828 filed Apr. 9, 1969 and now US. Pat. No. 3,556,067,.which application was a continuation-in-part of my application Ser. No. 629,488 filed Apr. 10, 1967, now abandoned.

SUMMARY OF THE INVENTION One object of the invention is to provide a high air velocity carburetor which substantially eliminates fuel vapor losses fuel to the chamber, and means operated by the throttle for opening the fuel valve.

Another object is to provide a carburetor in which the fuel reservoir or chamber is closed by a flexible diaphragm subject to pressures within and without the chamber and movable in response to differential pressure to open the fuel valve.

Another object is to provide a carburetor in which the diaphragm is movable by differential pressure independently of the operation of the throttle valve.

Another object is to provide a carburetor having an environmental temperature compensating control in the mechanical linkage between the throttle and the fuel valve.

Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawing wherein:

HO. 1 is a top plan view of a carburetor constructed in accordance with the invention.

FIG. 2 is a front view of the carburetor with parts shown in section and parts in elevation.

FIG. 3 is a fragmentary detail view corresponding to a portion of F K). 2 but illustrating a modification.

Referring now more particularly to the drawing and especially to FIGS. 1 and 2, the carburetor is shown as being of the down-draft type and comprises a housing 1 having a primary system 3 and a secondary system 4.

The primary and secondary systems 3 and 4 include the adjacent vertical bores in the housing defining the primary and secondary air intake passages 13F and 138. The passages 13? and 13S are separated by the housing wall 15 but converge below the wall in a common throttle bore 16 which has a conical entrance throat l8. Borel6 thus connects with and serves both the primary and secondary-systems, and is controlled by a suitably actuated throttle valve plate 19 therein pivoted on pin 19'. The throttle 19 is shown in solid lines at off-idle, and in dotted lines in part-open and wide-open positions.

The wall of primary passage 13F is provided intermediate its ends with an annular restriction forming a main venturi20P for the air flow. A booster venturi 21? of greater restriction than the main venturi 20? is disposed in the throat of the main venturi. A suitable choke plate 221 is pivotally mounted on pin 22' at the top or intake of the air passage 13? and is actuated in any well-known way for movement from the closed position shown in dotted lines to the open position shown in full lines.

The primary system also includes a fuel-receiving portion which has a fuel reservoir or chamber 2?. As indicated in FIG. 2, the fuel chamber 2P is a closed chamber being defined in part by a cavity in the housing and in part by a flexible diaphragm 5? which is clamped over an opening in the cavity by a cover plate 9?. The fuel chamber 2? is completely unvented and isolated from the outside atmosphere.

' Q the diaphragm 5?. The inlet 6? opens into the interior of the Fuel is admitted to the chamber 2? from a suitable fuel inlet 6? past a fuel control valve 7?. The fuel control valve 7P- includes a cylinder 7a having an elongated, axially movable needle valve 7b therein projecting through an orifice 7c. The needle valve 7b has a head 7d adapted to close the orifice and is normally urged to closed position by a calibrated spring AP bearing at one end against the head 7d and at the other end against the wall of the cylinder 7a. The needle valve 7b also has a stern 7e projecting from the head 7d and engageable with cylinder 7a but fuel from the inlet enters the chamber 2? only when the needle valve 7b is opened by movement of the diaphragm 5P to the left in FIG. 2.

The primary system 3 has a main jet 23? leading to fuel passages 24P and 25F. The passage 25? leads to the main fuel discharge orifice 32F and the passage 24? leads from the passage 25? to the

idle ports

27 and 28.

ldle ports

27 and 28 are open to manifold vacuum in the curb-idle position of the throttle plate 19 to provide the proper fuel mixture. The idle port 27 is controlled as to size by an idle adjustment screw 29.

The diaphragm SP is exposed to the pressure within the fuel chamber 2P on one side and to atmospheric pressure on the other side, and is movable in response to differential pressure. Movement of the diaphragm 5? to the left in FIG. 2 by differential pressure will move the needle valve 7b to the left and open orifice 70 so that fuel can flow into chamber 2?.

A mechanical linkage is provided to control the needle valve 7b in response to movement of the throttle 19. This linkage is indicated at 46? and includes a lever 47? pivoted at its upper end at 60? to the housing 1. Intermediate its ends, the lever has an abutment 61F engageable with, but not secured to; a member 62? on the outer side of the diaphragm 5?. At its lower end, the lever 47? is pivoted to the outer end of a push rod 481 which extends through an opening in a housing plate lb. The inner end of push rod 48? engages a cam 50? secured to and rotatable with throttle 19. A coil spring 64F compressed between the housing plate lb and a flange 63? on push rod 48? holds the inner end of the push rod in contact with the cam 50?. It will be apparent that rotation of the throttle plate 19 will rotate the cam 50] and since the inner end of rod 481 is in contact with the cam profile, the position of the throttle plate will determine the angular position of the lever 47].

With reference to the secondary system 4, a secondary throttle plate 36S is pivotally mounted on a pin 36' in the secondary air intake passage 13S adjacent the lower end.of wall 15. Throttle plate 368 is movable from the closed position shown in solid lines to the open position shown in dotted lines. The throttle plate 19 has a link 33S connected to it, and a link 355 is connected to the secondary throttle plate 36S. One end of a link 348 is pivoted to the link 3338 and the other end has a sliding pivotal connection in slot 395 in the link 3358 to provide a lost motion is the linkage system. An air valve 378 mounted on pivot pin 37 is disposed at the top of the secondary air intake passage 13$ for movement from the closed position shown in solid lines to the open position shown in dotted lines. There is an annular restriction forming a venturi 208 in passage 13S, and a booster venturi 218 in the throat of venturi 208.

During initial opening of throttle plate 19, secondary throttle plate 36S remains substantially closed due to the lost motion built into the linkage 33S-35S. At the intermediate position of throttle plate 19 shown in FIG. 2, called the pickup point," the linkage cause secondary throttle plate 368 to begin to open. By varying the construction of the linkage to throttle plate 36S, the rate of opening of plate 368 relative to plate 19 can be changed to provide different fuel-air mixtures at different loads and engine speeds to suit performance requirements.

As secondary throttle plate 368 opens, air valve plate37S is subjected to increased downward air pressure tending to open it from its generally closed position. As this occurs, additional air flow is generated through secondary chamber 13S and through bore 16. Depending upon the particular engine requirements, the responsiveness of plate 378 to air pressure, and thus the plate opening rate, can be controlled: In addition, it is preferable to prevent sudden snaplike opening of the plate 378. For this purpose control means are provided to govern or dampen the opening rate of the plate 378, such control means being shown as comprising a plurality of pressure relief openings 408 in the plate 378, which allow air to pass through at a predetermined calibrated rate. This reduces the initial force exerted on plate 378 when secondary throttle plate 368 is opened, and also provides a controlled initial rate of opening. Openings 408 are properly calibrated and disposed in the positions shown on an arc at progressively increased distances from the pivot pin 37'. My copending application Ser. No. 822,828, now U.S. Pat. No. 3,556,067 more fully describes such control means and also discloses another control means for plate 378.

Fuel is supplied to the secondary system from a fuel-receiving portion which is substantially the same as the fuel-receiving portion previously described and accordingly the same reference numerals with the suffix S instead of P are employed. Thus the fuel chamber 25 is defined in part by a cavity in the housing and in part by the flexible diaphragm 58 which is clamped over an opening in the cavity by the cover plate 95. The fuel chamber 28 is a completely closed reservoir unvented to the atmosphere.

The fuel control valve 78 controls the admission of fuel to the chamber 28 from the fuel inlet 68. The parts of this fuel valve 78 namely, the portions 7a-7e are the same as those previously described in connection with fuel valve 7? and function in the same manner, and the needle valve 7b is acted upon by a similar calibrated spring AS.

The secondary system 4 has a main jet 235 leading to fuel passages 24S and 25S. The passage 25S leads to the fuel discharge orifice 32S and the passage 24S leads from the passage 258 to the secondary idle port 518.

The diaphragm SS is exposed to the pressure within the fuel chamber 28 on one side and to atmospheric pressure on the other side, and is movable in response to differential pressure. Movement of the diaphragm 58 to the right in FIG. 2 by differential pressure will move the needle valve 7b of fuel valve 75 to the right and open orifice 70 so that fuel can flow into chamber 28.

The mechanical linkage 468 for controlling the needle valve of fuel valve 78 in response to movement of the throttle 19 is the same as the linkage 46? for controlling the needle valve of fuel valve 7?. This linkage includes the lever 47S pivoted at its upper end at 605 to the housing 1, which lever has an abutment 61S between its ends engageable with the member 628 on the outer side of the diaphragm 55. The lower end of lever 478 is pivoted to the outer end of a push rod 488 which extends through an opening in a housing plate lb. The inner end of push rod 485 engages a cam 508 secured to and rotatable with throttle 19. A coil spring 648 compressed between the housing plate lb and a flange 638 on the push rod 488 holds the inner end of the push rod in contact with the cam 508.

In operation, when the engine is turned over during starting, fuel under pressure enters the fuel inlet passages 61 and 6S and fills the passages completely up to fuel valves 7? and 73. The choke plate 22 is normally in a closed position when the engine is cold. The engine cranking induces a depression (vacuum) which is sensed at

idle ports

27, 28 and 51, as well as at the discharge 32?. The throttle plate 19 may be assumed to be in the idle position. This depression (vacuum) is transmitted through the main jets 23? and 238 to the

chambers

2P and 28. The depression (vacuum) in chambers 2? and 2S forces the diaphragms SP and 55 to move or flex inwardly pushing needle valves 7b inward against the tension of calibrated springs AP and AS, opening the orifices 7c and allowing fuel to flow from the inlets into

chambers

2P and 28. The fuel in chambers 2? and 25 is then transmitted through the main jets 23F and 238 to the respective passages 24?, 24S, 25? and 258. Thus fuel initially enters the air stream through

idle ports

27, 28, 51 and discharge 32?. Fuel does not flow initially from discharge 328 because both secondary throttle plate 368 and air valve plate 378 are closed at this time.

As the choke plate 22 opens, the depression (vacuum) at the main discharge 32F diminishes toward atmospheric pressure and fuel stops flowing through discharge 32?. At this point, the air passage 13P above throttle'plate 19 becomes substantially atmospheric, whereupon the discharge 32? becomes in effect an air bleed to idle passage 24? and leans out the idle fuel mixture to

ports

27 and 28. The main discharge 32?, when acting as an air bleed, being at or near atmospheric pressure, reduces or diverts the amount of vacuum that is available to be transmitted from idle channels 24? and 25P through the main jet 23F to chamber 2P. This reduction in depression (vacuum) reacts on the diaphragm SP and in turn the needle valve of fuel valve 7P reduces its opening orifice size due to the tension of calibration spring AP and thereby reduces the amount of fuel entering the chamber 2P.

At the curb idle position of throttle plate 19, the engine vacuum at

idle ports

27, 28 and 51 is high. This high depression (vacuum) is transmitted via the passages 24!, 245, 251 and 255 to the diaphragms SP and 5S and produces a corresponding movement thereof, The amount which the diaphragms SP and 58 move at curb idle is governed by engine depression only and is not controlled through the mechanical linkages 46F and 465.

As the throttle plate 19 is opened wider from curb idle, the movement of air through venturi 20F and venturi 2]? draws fuel through main discharge 32F. The depression (vacuum) in the fuel passages is further reduced and in turn the diaphragms SP and 58 move outward by differential pressure and the needle valves of the fuel valves 7? and 7S begin to close. However, the mechanical linkage 46F prevents the diaphragms from moving outward far enough to close the needle valves and maintains a constant linear relationship between the fuel requirements of the engine and the airflow through the carburetor at all positions of the throttle plate 19 other than curb idle, namely, off-idle, part throttle and wide-open throttle. Thus for a given throttle plate 19 position other than curb idle, a minimum needle valve position is maintained mechanically by the action of the linkage 46F and 46S upon the diaphragms. The amount of opening of the needle valves is governed by the profiles of cams SOP and 50S and the lever ratios of levers 47F and 47S. The cam profiles may be tailored to any desired fuel requirements.

The rate and amount of opening of the needle valves is transmitted mechanically from the cams SOP and 508 through the linkages to the outer or atmospheric sides of the diaphragms SP and 5S. These mechanical linkages provide the minimum fuel delivery necessary to operate the engine at the lowest hydrocarbon levels permissible. If the velocity of air passing discharge 32F and depression (vacuum) at

idle ports

27 and 28 should increase to an amount greater than normally encountered at a specific throttle 19 opening, the diaphragm SP is permitted to flex inwardly away from the lever 37P by differential pressure allowing a richer mixture in order to maintain a combustible fuel and air mixture to the engine maintaining a constant power output in relationship to air consumption demand. However, the differential pressure acting on the diaphragm cannot lean out or reduce the fuel delivery below that which the cam 50? is designed to deliver.

The secondary system functions in basically the same manner as the primary system except that its timing is governed by the point of opening of secondary throttle plate 368. However, the operation of the fuel valve 7S by differential pressure upon the diaphragm 5S and by operation of the mechanical linkage 468 through the action of cam 508 on throttle plate 19 is the same as that described for the primary system.

FIG. 3 shows a modified linkage 475' in which the push rod 488 extends into a thermal actuator 805 which has a chamber therein containing a thermally expansible material 815. The end of the push rod 488 is surrounded by an elastic boot 82S.

An increase in the ambient temperature causes the expansible material 818 in the chamber to expand and to force the push rod 485 outwardly to push the lever 47S away from diaphragm 55. The body of the thermal actuator 805 extends into a sleeve 838 secured to a cam follower 848 by a pin 858. The follower 848 extends into the sleeve 83S, and the thermal actuator body 808 and follower 848 have adjacent portions-of reduced cross section providing shoulders 86S and 878 against which the compression coil spring 888 bears to hold the follower 848 in contact with the cam 508.

It will be understood that a linkage similar to the linkage 468 may also be substituted in the primary system for the linkage 46?.

The temperature-compensated control provided by the linkage 46S and by a similar linkage for the primary system will decrease the stroke of levers 47? and 47S and hence the opening movement of the needle valves 7b with The in environmental temperatures and yet retain the desired fuel delivery characteristics determined by the profiles of cams SOP and 50S. Conversely, thetemperature compensated control provided by the modified linkages will increase the stroke of the levers with a drop in environmental temperature thereby enriching the fuel-air mixture in cold weather for maintaining uniform power output.

What I claim as my invention is:

l. in a carburetor, an air intake passage, a throttle valve controlling said air intake passage, a closed fuel-chamber, a fuel passage leading from said chamber to said air intake passage and transmitting vacuum in said air intake passage to said chamber, a fuel inlet passage to said chamber, a fuel valve controlling the flow of fuel from said fuel inlet passage to said chamber, means responsive to the movement of said throttle valve'for maintaining said fuel. valve open a predetermined minimum amount dependent upon the amount of opening of said throttle valve, and means responsive to the vacuum in said chamber for opening said fuel valve beyond said predetermined minimum amount to.a degree which is directly related to the magnitude of the vacuum in said chamber and which is independent of the opening of said throttle valve.

2. The structure defined in claim 1, wherein said means responsive to the movement of said throttle valve is operative to maintain a predetermined minimum opening of said fuel valve in all positions of said throttle valve except the idle position thereof.

3. The structure defined in claim 1, wherein said means responsive to the movement of said throttle valve includes a mechanical linkage.

4. The structure defined in claim 3, wherein said mechanical linkage includes a pivoted lever having ineans operable to open said fuel valve upon pivotal movement thereof in one direction, cam means on said throttle valve, and a member between said cam means and said lever for pivoting said lever in response to movement of said throttle valve.

5. The structure defined in claim 4, wherein said member comprises a push rod having one end connected to said lever and the other end engaging said cam means.

6. The structure defined in claim 5, wherein said linkage includes temperature-responsive means for changing the stroke of said lever in response to a change in temperature.

7. The structure defined in claim 1, comprising a secondary system having a secondary air intake passage opening into said first-mentioned air intake passage upstream of said throttle valve, a normally closed secondary throttle valve controlling said secondary air intake passage, a secondary closed fuel chamber, a secondary fuel passage leading from said secondary chamber to said secondary air intake passage and transmitting vacuum in said secondary air intake passage to said secondary chamber, a secondary fuel inlet passage to said secondary chamber, a secondary fuel valve controlling the flow of fuel from said secondary fuel inlet passage to said secondary chamber, means responsive to the movement of said first-mentioned throttle valve for maintaining said secondary fuel valve open a predetermined minimum amount dependent upon the amount of opening of said first-mentioned throttle valve, and means responsive to the vacuum in said secondary chamber for opening said secondary fuel valve beyond said predetermined minimum amount to a degree which is directly related to the magnitude of the vacuum in said secondary chamber and which is independent of the opening of said first-mentioned throttle valve, and means responsive to the movement of said first-mentioned throttle valve for opening said secondary throttle valve.

8. In a carburetor, an air intake passage, a throttle valve controlling said air intake passage, a closed fuel chamber, a fuel passage leading from said chamber to said air intake passage and transmitting vacuum in said air intake passage to said chamber, a fuel inlet passage to said chamber, a fuel valve controlling the flow of fuel from said fuel inlet passage to said chamber, a flexible diaphragm defining at least a portion of said chamber and subject to the vacuum within said chamber, said diaphragm being operatively connected to said fuel valve to open and close the latter, said diaphragm being movable in .one direction in response to an increase in the vacuum in said chamber to increase the opening of said fuel valve and movable in the opposite direction in response to a decrease in the vacuum in said chamber to decrease the opening of said fuel valve, abutment means engageable with said diaphragm for limiting movement of said diaphragm in said opposite direction, said diaphragm being movable in said one direction free of said abutment means, and means operated by said throttle valve for positioning said abutment means in accordance with the opening of said throttle valve.

9. The structure defined in claim 8, wherein said abutment means and last-named means are operative to maintain a predetermined minimum opening of said fuel valve in all positions of said throttle valve except the idle position thereof.

10. The structure defined in claim 9, wherein said lastnamed means includes a mechanical linkage having a pivoted lever on which said abutment means is mounted, a cam on said throttle valve, and a push rod having one end connected to said lever and the other end engaging said cam.

11. The structure defined in claim 10, wherein said linkage includes temperature-responsive means for decreasing the stroke of said lever in response to a rise in temperature.

12. In a carburetor, primary and secondary systems having a primary air intake passage, a primary throttle valve controlling said primary air intake passage, a secondary air intake passage opening into said primary air intake passage upstream of said primary throttle valve, a normally closed secondary throttle valve in said secondary air intake passage, primary and secondary closed fuel chambers, primary and secondary fuel passages respectively leading from said primary and secondary chambers to said primary and secondary air intake passages and transmitting vacuum in the latter to the former, primary and secondary fuel inlet passages to said respective primary and secondary chambers, primary and secondary fuel valves controlling the flow of fuel respectively from said primary and secondary fuel inlet passages to said primary and secondary chambers, primary and secondary flexible diaphragms defining at least a portion of said respective primary and secondary chambers and subject to the vacuums therewithin, said primary and secondary diaphragms being operatively connected to said primary and secondary fuel valves respectively to open and close the latter, each diaphragm being movable in one direction in response to an increase in the vacuum in .the associated chamber to increase the opening of the associated fuel valve and movable in the opposite direction in response to a decrease in the vacuum in the associated chamber to I decrease the opening of the associated fuel valve, abutment means engageable with each diaphragm for limiting the movement thereof in said opposite directions, said diaphragms being movable in said one direction free of the associated abutment means, means operated by said primary throttle valve for positioning each said abutment means in accordance with the opening of said primary throttle valve, and means responsive to the movement of said primary throttle valve for opening said secondary throttle valve.

pivoted lever on which said one abutment means is mounted, a cam on said throttle valve, and a push rod having one end connected to said lever and the other end engaging said cam, said linkage including temperature-responsive means for decreasing the stroke of said lever in response to a rise in temperature.

Claims

1. In a carburetor, an air intake passage, a throttle valve controlling said air intake passage, a closed fuel chamber, a fuel passage leading from said chamber to said air intake passage and transmitting vacUum in said air intake passage to said chamber, a fuel inlet passage to said chamber, a fuel valve controlling the flow of fuel from said fuel inlet passage to said chamber, means responsive to the movement of said throttle valve for maintaining said fuel valve open a predetermined minimum amount dependent upon the amount of opening of said throttle valve, and means responsive to the vacuum in said chamber for opening said fuel valve beyond said predetermined minimum amount to a degree which is directly related to the magnitude of the vacuum in said chamber and which is independent of the opening of said throttle valve.

4. The structure defined in claim 3, wherein said mechanical linkage includes a pivoted lever having means operable to open said fuel valve upon pivotal movement thereof in one direction, cam means on said throttle valve, and a member between said cam means and said lever for pivoting said lever in response to movement of said throttle valve.

8. In a carburetor, an air intake passage, a throttle valve controlling said air intake passage, a closed fuel chamber, a fuel passage leading from said chamber to said air intake passage and transmitting vacuum in said air intake passage to said chamber, a fuel inlet passage to said chamber, a fuel valve controlling the flow of fuel from said fuel inlet passage to said chamber, a flexible diaphragm defining at least a portion of said chamber and subject to the vacuum within said chamber, said diaphragm being operatively connected to said fuel valve to open and close the latter, said diaphragm being movable in one direction in response to an increase in the vacuum in said chamber to increase the opening of said fuel valve and movable in the opposite direction in response to a decrease in the vacuum in said chamber to decrease the opening of said fuel valve, abutment means engageable with said diaphragm for limiting movement of said diaphragm in said opposite direction, said diAphragm being movable in said one direction free of said abutment means, and means operated by said throttle valve for positioning said abutment means in accordance with the opening of said throttle valve.

10. The structure defined in claim 9, wherein said last-named means includes a mechanical linkage having a pivoted lever on which said abutment means is mounted, a cam on said throttle valve, and a push rod having one end connected to said lever and the other end engaging said cam.

12. In a carburetor, primary and secondary systems having a primary air intake passage, a primary throttle valve controlling said primary air intake passage, a secondary air intake passage opening into said primary air intake passage upstream of said primary throttle valve, a normally closed secondary throttle valve in said secondary air intake passage, primary and secondary closed fuel chambers, primary and secondary fuel passages respectively leading from said primary and secondary chambers to said primary and secondary air intake passages and transmitting vacuum in the latter to the former, primary and secondary fuel inlet passages to said respective primary and secondary chambers, primary and secondary fuel valves controlling the flow of fuel respectively from said primary and secondary fuel inlet passages to said primary and secondary chambers, primary and secondary flexible diaphragms defining at least a portion of said respective primary and secondary chambers and subject to the vacuums therewithin, said primary and secondary diaphragms being operatively connected to said primary and secondary fuel valves respectively to open and close the latter, each diaphragm being movable in one direction in response to an increase in the vacuum in the associated chamber to increase the opening of the associated fuel valve and movable in the opposite direction in response to a decrease in the vacuum in the associated chamber to decrease the opening of the associated fuel valve, abutment means engageable with each diaphragm for limiting the movement thereof in said opposite directions, said diaphragms being movable in said one direction free of the associated abutment means, means operated by said primary throttle valve for positioning each said abutment means in accordance with the opening of said primary throttle valve, and means responsive to the movement of said primary throttle valve for opening said secondary throttle valve.

13. The structure defined in claim 12, wherein said last-mentioned means includes lost motion means to delay the opening of said secondary throttle valve during initial opening of said primary throttle valve.

14. The structure defined in claim 13, wherein the means operated by said primary throttle valve for positioning one of said abutment means includes a mechanical linkage having a pivoted lever on which said one abutment means is mounted, a cam on said throttle valve, and a push rod having one end connected to said lever and the other end engaging said cam, said linkage including temperature-responsive means for decreasing the stroke of said lever in response to a rise in temperature.