US3051150A

US3051150A - Automatic-spark advance mechanism

Info

Publication number: US3051150A
Application number: US86080A
Authority: US
Inventors: Kalin S Johnson; Robert K Frank
Original assignee: Holley Carburetor Co
Current assignee: Holley Performance Products Inc
Priority date: 1961-01-31
Filing date: 1961-01-31
Publication date: 1962-08-28
Anticipated expiration: 1979-08-28

Description

1962 K. s. JOHNSON ETAL 3,051,150

AUTOMATIC SPARK ADVANCE MECHANISM Filed Jan. 51, 1961 5 Sheets-Sheet l INVENTORS K41. //v 5. Jd /A/SO/V &

05 527 /C Fem K MAN Aug. 28, 1962 K. s. JOHNSON ETAL 3,051,150

AUTOMATIC SPARK ADVANCE MECHANISM Filed Jan. 31, 1961 5 Sheets-Sheet 2 Aug. 28, 1962 K. s. JOHNSON ET AL AUTOMATIC SPARK ADVANCE MECHANISM s Sheets-Sheet 5 Filed Jan. 31, 1961 INVENTORS K4 L//\/ 5 JbH/VSO/V 2 BY/POBE/PT F/PA/VK Q 031% ATTORNEY res Parent 3,951,158 Patented Aug. 28, 1962 This invention relates generally to ignition distributors for internal combustion engines, and more specifically to those ignition distributors wherein the advance mechanism is controlled in accordance with engine manifold and carburetor venturi vacuums, commonly referred to as full pressure distributors.

Full pressure distributors usually employ a flexible diaphragm, which is secured to a movable breaker plate and positioned by both the eng'ne manifold vacuum and venturi vacuum. Springs are also provided for purposes of calibration by resisting the movement of the diaphragm.

In order to obtain smooth engine idling, maximum retardation of the ignition is usually necessary while slight opening of the throttle requires some advance of the ignition timing. This general proposition, however, does not hold true for all engine operating conditions. That is, for example, during heavy load, low speed engine operating conditions, a retard of the ignition timing is desirable in order to prevent undesirable detonation.

Accordingly, in order to accomplish the above, it has become accepted practice to provide what is commonly referred to as a spark valve in cooperation with the ignition distributor and carburetor. The spark valve is usually comprised of a valve member positioned in accordance with vacuum responsive means so as to at times allow communication between a source of engine manifold vacuum and the flexible diaphragm of the ignition distributor.

The vacuum responsive means is made sensitive to the manifold vacuum so as to allow the above communication only after a predetermined minimum value of manifold vacuum has been attained. The accepted procedure in determining the value of manifold vacuum is by the provision of a pre-loaded spring which is so placed as to oppose the movement of the vacuum responsive means in the valve opening direction.

Ignition systems constructed in accordance with the above are, nevertheless, still deficient in at least one respect. That is, once the value of the manifold vacuum for opening the spark valve has been selected, there is no way of changing the point at which the spark valve will open. In other words, by selecting one such value, optimum engine performance is often sacrificed in at least some areas of the overall engine operating range.

Accordingly, it is now proposed to provide means for varying the predetermined value of manifold vacuum required for opening the spark valve in accordance with engine load.

More specifically an object of this invention is to provide means responsive to throttle valve position for varying the point at which the spark valve will allow communication between a source of engine manifold vacuum and flexible diaphragm which is the immediate control over the ignition timing.

Other objects and advantages will become apparent when reference is made to the following written description and accompanying drawings wherein:

FIGURE 1 is a fragmentary cross-sectional view of a carburetor constructed generally in accordance with the teachings of the invention and cooperating with an ignition distributor;

FIGURE 2 is a graph illustrating the relationship between spark vacuum and engine speed;

FIGURE 3 is a graph illustrating the relationship between degrees of ignition advance and engine speed;

FIGURE 4 is a fragmentary cross-sectional view of a modification of the invention;

FIGURE 5 is a cross-sectional view, partially illustrated in schematic form, of another modification of the invention.

Referring now in greater detail to the drawings, FIG- URE 1 illustrates a carburetor 10 having an induction passage 12 with a throttle valve 14 therein controlling the flow therethrough to the intake manifold 16 of the engine. An actuating lever 18 may be suitably secured to the throttle shaft 22, and a rod 20 and spring 24 may be provided to cooperate with lever 18 for positioning the throttle valve.

The ignition distributor 26 may be comprised generally of a housing 28 having provision therein for the reception of a driving shaft to which a cam 30 is secured so as to rotate therewith. A breaker plate 32 mounted on a suitable bearing member is adapted to rotateabout the driving shaft and cam 30. The breaker plate 32 carries thereon a breaker arm assembly 34 which includes a cam engaging portion 36 'engageable by the rotating cam 30 to efiect the make and break of the ignition circuit at the contacts indicated at 38. The cam 30 and the shaft on which it is mounted are, of course, rotated in timed relation with the engine by means of any transmission means well known in the art.

The distributor advance mechanism is controlled generally by the pressure responsive device 40 which commum'cates by means of suitable conduitry with

ports

42 and 44 in the carburetor 10 so as to sense engine manifold and carburetor venturi vacuum, respectively.

The pressure responsive device 40 may be substantially comprised of a housing 46, and a cover member 48 rigidly held together in a manner so as to peripherally secure a diaphragm member 50 therebetween. A member 52, connected at its one end to the diaphragm member 50', is adapted tobe operatively engaged with the breaker plate 32, as by means of a hole formed therein for the reception of the end 54 of member 52. Any suitable retaining means suchas a cotter pin 56 may be employed so as to prevent accidental disengagement between member 52 and the breaker plate 32. Chamber 58 formed generally by "cover 48 and diaphragm 50 is exposed to a. source of vacuum, while the chamber 60 is vented to the atmosphere. The entire pressure responsive device may, of course, be secured to the housing 28 by any suitable means such as screws 62.

Calibration springs 64 and 66 are secured at one end to the

anchor pins

68 and 70, which in turn are fixedly secured to the housing 28. The other ends of springs 64 and 66 are connected to the breaker plate spring posts 72 and 74. Openings 76 and 78 may of course be formed in the breaker plate 32 in order to provide the necessary clearance for

anchor pins

68 and 70.

Other portions of the distributor such as the distributing rotor, upper distributor cap and various output terminals are neither shown nor described, since they form no part of this invention and are well known to those skilled in the art.

Valving means illustrated generally at 80 are provided in order to control the engine manifold vacuum which is ultimately communicated to chamber 58 through restriction 144. The Valving means is comprised of two independently actuated

valve assemblies

82 and 84, which may be contained within the general structure of the carburetor body '11 if so desired.

Valve assembly 84, which may be threadably received within the chamber 88, is comprised of a housing portion 104 which cooperates with a cover member 106 to pcripherally secure a pressure responsive diaphragm memher 108 therebetween. Chamber 110, formed generally by the diaphragm 108 and cover 106 is ventedto the atmosphere, as by ports 112, while chamber 114 on the opposite side of the diaphragm isin communication with the induction passage 12 by means of port 42, conduit 116 and radially formed passages 118 in the housing '104.

An orifice 120 formed in housing 104 provides communication at times betweenchambers 88 and 114, depending upon the position of valve member 122, which has its one end 124 connected to'the diaphragm 108 so 'as to be moved therewith. Spring 126 and adjusting member 128 are provided so asto enable selection of the particular vacuum value at which valve member 122 will be moved to the left and orifice 120 will be opened.

The throttle shaft 22 has a cam member 90 secured thereto which is adapted to be in constant contact with end 92 of the valve member 94 which comprise valve assembly 82. Valve member 94 is s'lidably received Within body portion 1 1 and is urged to the left by both

springs

126 and 96. An adjustable member 98 is also provided in order to vary the initial force of spring 96..

The other end of member 94 has a spring abutment portion 130 formed thereon against which spring 126 normally urges. Additionally, a passage 132 is provided so as m at times allow communication between chamber 88 and conduit 134 which is in communication with conduit 116. A restriction 136 may be provided in conduit 116 while restriction 138is provided in conduit 140 which continually communicates between chamber 88 and conduit 116.

A conduit 142, including a restriction 144 therein, communicates between chamber 88 and conduit 146 which leads from port 44 to chamber 58 of the pressure responsive device 40.

Operation Referring to FIGURE 1, let it be assumed that the engine is running at curb idle and the spark advance mechanism is in full retard. Under these conditions, the elernents would assume positions as illustrated in FIG- URE 1. r

Springs 64 and 66, being anchored at one end to the housing 28 by means of

anchor pins

68 and 70, respectively, and connected to their respective other ends to the breaker plate spring posts 72 and 74, urge the breaker plate 32 clockwise about the distributor shaft and cam 30 until the holes 76 and 7 8 abut against the

anchor pins

68 and 70. This causes the breaker arm assembly 34 to be rotated to a position whereby the cam 30 will contact the cam engaging portion 36 at a time when the respective pistons within the engine are most nearly at top dead center.

If the throttle valve 14 is moved to wide open position, the air flow past the venturi 148 causes a venturi vacuum at port 44 which is directed generally by means of conduit 146 to chamber 58. As engine speed increases with the throttle valve maintained at wide open position, the venturi vacuum increases, causing the diaphragm 50 to be moved in ever increasing amounts to the left thereby rotating the breaker plate 32 and the breaker arm assembly counter-clockwise about cam 30. This in turn causes the cam 30 to engage the cam engaging portion 36 at ever increasing degrees of engine crankshaft rotation ahead of when the engine pistons reach top dead center. This is 'usually referred to by those skilled in the art as degrees of ignition advance.

It should be noted that during this period of wide open throttle operation, the throttle shaft 22, by virtue of the action of its cam portion 90 during the last few degrees of movement towards wide open throttle position, has allowed'valve member 94 to be moved to the left by the cooperative action of

springs

96 and 126. As a consequence a results in a bleedingreffect causing a diminution of the 'venturi vacuum as sensed by diaphragm 50 and continuing to do so whenever the valve member 94 has placed passage 132 in juxtaposition with conduit 134.

In view of the above, itbecomes apparent that lighter calibrating springs 64 and 66 may be employed; this is because the eifective force opposing them and created by the venturi vacuum has been materially diminished by causing passage 132 to communicate between

conduits

134 and 142 so as to allow atmospheric pressure to bleed off the venturi vacuum.

If, for example, the throttle valve were opened slowly from its fully closed position to some part throttle position as required by normal road loads, the valve member 94 would not have moved sufliciently to allow communication through passage 132 and po1t'42 would be exposed to manifold vacuum. I

The existing manifold vacuum would then be communicated by means of

conduits

116 and 118 to chamber 114, causing the diaphragm 138 and valve 122 to move to the left. Since, as stated previously, the passage 132 is closed, the manifold vacuum is directed through restriction 144 to chamber 58 causing an increase in ignition advance. As a result of the provision of lighter or weaker calibrating springs, there is less resistance oifered to the movement of the diaphragm 50, and consequently greater ignition advances at part throttle engine operating conditions is obtainable than with prior conventional ignition systems.

A separate conduit 149, including a restriction 138 therein, may be provided so as to continually communicate a small degree of manifold vacuum to conduit 142 during those periods of engine operation wherein the manifold vacuum as yet is insufficient to fully open valve 122 against the action of spring 126. The basic operation of the invention is the same; the primary distinction is that in the absence of apassage such as conduit 144), no communication of manifold vacuum from port 42 to conduit 142 is possible until that predetermined value of manifold vacuum is attained which will completely open valve 122.

Another important feature of the invention is the provision of means whereby the predetermined value of manifold vacuum necessary to open valve 122 can be varied in accordance with engine load. This is accomplished generally by the cooperative action of valve assembly 84 and

members

22, and 94.

For example, with the elements in the positions illustrated by FIGURE 1, spring 126 may possibly be compressed so as to exert a force of one pound in a direction closing the valve 122, whereas when the throttle 14 is moved to half wide-open position the force of spring 126 may be reduced to possibly 0.75 pound. The exact forces are of course a matter of design and the variation as between any two throttle positions will be determined by the particular contour employed on cam member 90.

' Consequently, since the force of spring 126 can be varied in accordance with throttle valve position, that is, engine load, it becomes apparent that at certain times a lesser pressure difierential across diaphragm 1118 is required to open valve 122 than at other times. This of course means that, depending on engine load or throttle valve position, the valve 122 will open at infinite numbers of distinctly diiferent values of manifold vacuum.

FIGURE 2 is a graph which illustrates the relationship between spark vacuum and engine speed, By spark vacuum is meant the actual vacuum sensed by the diaphragm member 50. Curves B through G are obtained by running the engine at a constant manifold vacuum and the numbers, 2 through 14, on curves B through G represent the manifold vacuum value in terms of inches of mercury (Hg). Curve Ais of course a low variable manifold vacuum curve, that being obtained by running the engine at wide openthrottle (W.O'.T.).

The second set of curves labeled H through L represent the spark vacuum obtainable at different fixed throttle openings wherein the load on the engine is varied in order to increase or decrease engine speed. The numbers through 60 represent the degrees of throttle opening from closed throttle position.

As was stated previously, usually during periods of heavy load, low speed engine operation a retard of the ignition timing is desirable in order to prevent undesirable detonation. The conventional distributor would have spark advance curves similar to curves R, S, T, U, and V, of FIGURE 3 which result in ignition advances of approximately 6, 13 18, 22, and 27, respectively at 900 rpm. However, in a particular engine it may be determined that at 10 throttle opening, at an engine speed of approximately 900 rpm. and at 20 throttle opening, at an engine speed of 1400 rpm. and at 40 throttle opening, at an engine speed of 1900 r.p.m. no ignition advance due to manifold vacuum is desired because such would result in undesirable detonation. in other words, the combinations represented beneath and to the left of line X of FIGURE 2 are undesirable.

The invention now provides means whereby the above can be accomplished. That is, the throttle valve position is indicated to the valve assembly 84 by means of cam 90, member 94 and spring 126. It should be noted that at closed throttle position the force of spring 126 closing valve member 122 is the greatest and that progressive opening of the throttle results in the lessening of the spring force on valve 122. This in turn determines when valve 122 will open and communicate the then-existing manifold vacuum to diaphragm member 50.

Consequently, by properly contouring the cam member 90, it is possible to obtain ignition advance curves represented by curves M, N, O, P, Q and R or" FIGURE 3 instead of those represented by S, T, U and V. It should be noted that at heavy load, low engine speed operation no appreciable ignition advance due to manifold Vacuum is realized.

As was previously stated, conduit 14% and restriction 133 may be provided as illustrated if desired. However, its use is not necessary in order to practice the invention. It a conduit similar to conduit 14% is not employed, no communication of manifold vacuum from port 42 to conduit 142 is possible until that predetermined value of manifold vacuum is attained which will completely open valve 122, against the force of spring 126.

Just as conduit 149,

so'may conduits

132 and 134 be eliminated along with restriction 136. This construction is illustrated by FIGURE 4. All elements which are like or similar to those of FIGURE 1 are identified with like reference numbers. It should be noted that element 94 does not contain a passage 132 as previously illustrated in FIGURE 1, nor does a passage functionally similar to conduit 134 exist. The sole purpose of element 94, in the embodiment of FIGURE 4, is to vary the force of spring 126.

However, the function of

conduits

132 and 134, as previously described, can nevertheless be retained by the provision of spring retainers 158 and 161 That is, the cam 90 may be contoured so as to allow, at substantially wide-open throttle conditions, member 94 to be moved sufficiently to the left by spring 96 so as to open valve 122. In other words, member 94 Will move a sufficient amount to place the normally compressed spring 126 in somewhat of a state of tension thereby pulling valve 122 open. 1

The embodiment disclosed by FIGURE 5 is structurally similar and functionally identical to that disclosed by FIGURE 4. All elements which are like or similar to those of FIGURES 1 and 4 are identified with like reference numbers. Member 150, which is in contact with cam member 90, has a threaded portion thereon adapted to receive a coacting spring abutment member 152. The spring 154, placed intermediate member 152 and diaphragm 108, transmits the movements of cam 90 as a force opposing the force ofspring 126. That is, assuming that spring 126 has a force of 1.0 lb. on valve 122 in the position shown, it becomes evident that as cam 98 is rotated in the throttle opening direction that spring 154 is caused to be increasingly compressed and that the net efifective force of spring 126 tending to keep valve 122 closed, has been diminished by an amount equal to the increase in compression of spring 154.

Member 151? may of course be constructed of a length suitable for positively opening the valve 122 against the force of spring 126. That is, during periods of, for example, wide open throttle operation, it may be desirable to fully open valve 122. Accordingly, this can be accomplished by having end 156 of member abut against member 124 in a manner so as to forcibly move the valve 122 to the right thereby opening conduit 121) and allowing communication between

conduits

116 and 142.

Although but three embodiments of the invention have been disclosed and described, it is apparent that other modifications of the invention are possible within the scope of the appended claims.

What we claim as our invention is:

1. An ignition system for an internal combustion engine having an induction passage with a throttle valve controlling the flow of air therethrough, comprising an ignition distributor having a housing, a switch in said housing, a cam member normally rotatable in accordance with engine speed and adapted to intermittently open said switch for effecting a make and break of the ignition circuit, a breaker plate normally supporting said switch and having relative angular motion with respect to said housing, pressure responsive means secured to said housing and operatively connected to said breaker plate for rotating said breaker plate in accordance with engine requirements, a venturi formed in said induction passage upstream of said throttle valve, first conduit means communicating between the throat of said venturi and said pressure responsive means, second conduit means communicating between said induction passage at a point upstream of said throttle valve when said throttle valve is in its normally closed position and said first conduit means, first valve means connected serially in said second conduit means and adapted to at times control the communication through said second conduit means, said first valve means comprising a diaphragm member, a valve member secured to said diaphragm member and adapted to be positioned thereby, said diaphragm member being responsive to the vacuum sensed by said second conduit means at its point of communication with said induction passage and effective to move said valve member in the opening direction upon the attainment of a predetermined vacuum value, third conduit means connected in parallel with said second conduit means so as to bypass said first valve means, a variably positioned abutment member, spring means connecting said abutment member and said valve member to each other, a cam member rotatably positioned in accordance with the position of said throttle valve, second spring means continually urging said abutment member against said cam member, said cam member and abutment member being eflfective to vary the force of said first mentioned spring means so as to vary said predetermined vacuum value required to open said valve member.

2. An ignition system for an internal combustion engine having an induction passage with a throttle valve controlling the flow of air therethrough, comprising an ignition distributor having a housing, a switch in said housing, a cam member normally rotatable in accordance with engine speed and adapted to intermittently open said switch for efiEecting a make and break of the ignition circuit, a breaker plate normally supporting said switch and having relative angular motion with respect to said housing; pressure responsive means secured to said housing and operatively connected to said breaker plate for rotating said breaker plate in accordance with engine requirements, a venturi formed in said induction passage upstream of said throttle valve, first conduit means communicating between the throat of said venturi. and said pressure responsive means, second conduit means communicating between said induction passage at a point upstream of said throttle valve when said throttle valve is in its normally a variably positioned abutment member, spring means connecting said abutment member and said valve member to each other, a cam member rotatably positioned in accordance with the position of said throttle valve, second spring means continually urging said abutment member against said cam member, said cam member and abutment member being efiective to vary the force of said first mentioned spring means so as to vary said predetermined vacuum value required to open said valve member.

3. An ignition system for an internal combustion engine having an induction passage with a throttle valve controlling the flow of air therethrough, comprising an ignition distributor having a housing, a switch in said housing, a cam member normally rotatable in accordance with engine speed and adapted to intermittently open said switch foreiliecting a make and break of the ignition circuit, a breaker plate normally supporting said switch and having relative angular motion with respect to said housing, pressure responsive means secured to said housing and operatively connected to said breaker plate for rotating said breaker plate in accordance with engine requirements, a venturi formed in said induction passage upstream of said throttle valve, first conduit means communicating between the throat of said venturi and said pressure responsive means, second conduit means communicating between said induction passage at a point upstream of said throttle valve when said throttle valve is in its normally closed position and said first conduit mined vacuum value, third conduit means connected in parallel with said second conduit means so as to bypass said first valve means, a variably positioned abutment member, spring means connecting said abutment member and said valve member to each other, a cam member rotatably positioned in accordance with theposition of said throttle valve, second spring means continually urging sadi abutment member against said cam member, said cam member and abutment memberbeing effective to vary the force of said first mentioned spring means so as to vary said predetermined vacuum value required to open said valve member, fourth conduit means having its one end in continual communication with said second conduit means at a point between said first valve means and said induction passage, second valve means for at times connecting the other end of said fourth conduit means to said second conduit means so as to bypass said first valve means, and means responsive to the position of said throttle valve and operative during periods of substantially wide open throttle operation for causing said 8 second valve means to complete the communication between said other end of said fourth conduit means and said second conduit means. 7

4. An ignition system for an internal combustion engine having an induction passage with a throttle valve controlling the flow of air therethrough, comprising an ignition distributor having a housing, a switch in said housing, a cam member normally rotatable in accordance with engine speed and adapted to intermittently open said switch for efiecting a make and break of the ignition circuit, a breaker plate normally supporting said switch and having relative angular motion with respect to said housing, pressure responsive means secured to said housing and operatively connected to said breaker plate for rotating said breaker plate in accordance with engine requirernents, a venturi formed in said induction passage upstream of said throttle valve, first conduit means communicating between the throat of said venturi and said pressure responsive means, second conduit means commuriicating between said induction passage at a point upstream of said throttle valve when said throttle valve is in its normally closed position and said first conduit means, first valve means connected serially in said second conduit means adapted to at times control the communication through said second conduit means, said first valve means comprising a diaphragm member, a valve member secured to said diaphragm member and adapted to be positioned thereby, said diaphragm member being responsive to the vacuum sensed by said second conduit means at its point of communication with said induction passage and effective to move said valve member in the opening direction upon the attainment of a predetermined vacuum value, a variably positioned abutment member, spring means connecting said abutment member and said valve member to each other, a cam member rotatably positioned in accordance with the position of said throttle valve, second spring means continually urging said abutment member against said cam member, said cam member and abutment member being efiective to vary the force of said first mentioned spring means so as to Vary said predetermined vacuum value required to open said valve member, third conduit means having its one end in continual communication with said second conduit means at a point between said first valve means and said induction passage, second valve means for at times connecting the other end of said third conduit means to said second conduit means so as to bypass said first valve means, and means responsive to the position or said throttle valve and operative during periods of substantially wide open throttle operation for causing said second valve means to complete the communication between said other end of said third conduit means and said second conduit means.

5. In an ignition system for an internal combustion engine having an induction passage with a throttle valve controlling the flow of air therethrough, the combination of an ignition distributor including timing means therein for controlling the timing of ignition, vacuum responsive means for positioning said timing means, means for creating a first vacuum in accordance with the velocity of air flow through said induction passage, means for transmitting said first vacuum to said vacuum responsive means, engine vacuum responsive valve means for at times directing engine vacuum to said vacuum responsive means,

resilient means biasing said valve means in the closing direction, and variably positioned abutment means positioned in accordance with the position of said throttle valve forvarying the force of said resilient means.

6. in an ignition system for an internal combustion engine having an induction passage with a throttle valve controlling the fiow of air therethrough, the combination of an ignition distributor including timing means therein for controlling the timing of ignition, vacuum responsive means for positioning said timing means, means for creating a first vacuum in accordance with the velocity of air flow through said induction passage, means for transmitting said first vacuum to said vacuum responsive means, engine vacuum responsive valve means for at times directing engine vacuum to said vacuum responsive means, resilient means biasing said valve means in the closing direction, and means responsive to the position of said throttle valve for increasing the closing force of said resilient means as said throttle valve approaches a substantially closed position.

7. In an ignition system for an internal combustion engine having an induction passage With a throttle valve controlling the flow of air therethrough, thet combination of an ignition distributor including timing means therein for controlling the timing of ignition, vacuum responsive means for positioning said timing means, means for creating a first vacuum in accordance With the velocity of air flow through said induction passage, means for transmitting said first vacuum to said vacuum responsive means, engine vacuum responsive valve means for at times directing engine vacuum to said vacuum responsive means, resilient means biasing said valve means in the closing direction, and variably positioned abutment means positioned in accordance with the position of said throttle valve for varying the force of said resilient means, said resilient means being connected to said abutment means and said valve means in such a manner so as to be capable of being placed in a state of compression at certain periods of operation While being placed in a state of tension during other periods of operation.

it) 8. In an ignition system for an internal combustion engine having a passage with the throttle valve controlling the flow of air therethrough and an ignition distributor,

a control valve assembly having timing means therein for controlling the timing of the ignition and pressure responsive means for positioning said timing means, said assembly comprising a first vacuum means for creating a vacuum in accordance with the rate of air consumption of said internal combustion engine, conduit means for communicating said first vacuum to said pressure responsive means, means for reducing the efiect of said vacuum on said pressure responsive means, second vacuum means responsive to a predetermined value of engine vacuum for communicating said engine vacuum to said pressure responsive means, and means associated With said throttle valve and responsive to engine load for variably determining said predetermined value of engine vacuum at which said second vacuum means becomes operative for communicating said engine vacuum to said pressure responsive means.

References Cited in the file of this patent UNITED STATES PATENTS 2,381,610 Mallory Aug. 7, 1945 2,473,805 Mallory June 21, 1949 2,475,717 Ostling July 12, 1949 2,867,197 McCullough Ian. 6, 1959