CA1041853A - Thermostatic automatic choke control for small engines - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In a dual-range thermostatic device comprising a control shaft that is rotatable between defined hot and cold positions, shaft position is established by two spirally coiled bimetal thermostats, one for high temperatures, one for low temperatures, such in a unidirectional torque transmitting con-nection between the shaft and relatively fixed structure where-by each thermostat imposes force upon the shaft only at temperatures within its own range. One bimetal thermostat exerts increasing force between its end portions in response to in-creasing temperatures, the other, in response to decreasing temperatures. The relatively fixed structure comprises tele-scoped cylinders having interengaging splined serrations en-abling them to be established in any of several different rotational positions, for adjustment of the temperatures at which the shaft is brought to its defined positions.

Description

10~8S3 Th~ application i8 closely relat~d ~n subject matter to our copending Canadian application, Ser~al No 192,055, filed February 8, 1974, and its divis~onal appl~cation Serial ~o 243,392 , f~led January 14, ~976 This invention relate~ broàdly to ~nternal com--bustion engines, especially engin~s o~ the sm_ll single-cylind-r air-cooled type, and more partioularly to an automatic control for the carburetor choke v~lve of such an engino.
In the automatio chokc control device o~ the present ~nvention, A choke v_lve iB biased towar~s its clo~ed pos~tion and can bo 8wung towards it~ op~n po~ition by mcans of an air vane whioh iB respon~ivc to the stroam of engine cooling air that iB generat-d by A blowor flywheel on the en- --gine Swinging o~ the air v_no, and hence of the choke valve, ~-~ controlled by a thermostat which has a lost motion connec- ~
tion with the air vane ~nd which i~ located to be influenced - -by thc heat of the ongine~ ~
In the apparAtu~ of the pr~sent invention, the ~;
a~r v_ne and tho th-rmostat aro r-spectively loc~ted at oppo-site ond portions of a singlo rotAtably unted control ~ha~t The th-rmostat i8 seAted in _ woll formed in the cylinder c~st~
~ng, c10801y ad~accnt ta the ongin~ exhaust port, where th~
in~luenc- o~ engine he~t upon it i8 as6urcd The a~r vane i~ -in the ~tre~ of cooling air generated by th- convcn~ional ~ -blower flywheal on the cnginst under the shroud that gu~de~ the coo~ing Air across the cylinder body The control shaft, whSch compr~e~ a lost tion connection bctween the thermostat and the d r ~ane, ~ ~ikew~se ~n an out-o~-the way loc~tion, mo~t o~ lt be~ng under the blower shroud A6 compared w~th prior au;omatic choke control do~cc~, th~t o~ th~ pr--ant ~nvention pro~ide~ much more ro-l~ablc und accurato ¢hoke ~alve positioning, throughout a ~ub~ant~ally w~d-r r~ngo o- amb~ent ~nt ongine t~mporature ,~
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conditions, by reason of its comprising two bimetal elements, one of which is effective at high temperatures, the other at low temperatures.
The conventional thermostat heretofore employed in automatic choke control devices comprised a single flat bi-metal strip that was coiled into a spiral or a helix. One end of the strip was confined against motion; its other end was attached to a rotatable control member. When the thermostat was subjected to a changing temperature, the control member was rotated in a direction that depended upon whether the tem-perature change caused the bimetal strip to expand and unwind or to contract and curl up.
When an internal combustion engine is running and fully warmed up, its automatic choke control thermostat is subjected to a temperature in excess of 400F. In a cold engine the thermostat is of course at ambient temperature, which can be anywhere within the range of, say, 20F. below zero to 80F. or more above zero. This means that for choke control that will afford easy starting under all expectable weather con-ditions, a thermostat that cooperates with an air vane choke actuator must be capable of fully withstanding the choke open- -~
ing force exerted by the actuator at the lowest expectable ambient temperature but must permit the choke actuator to take -over and swing the choke open little by little as the engine -temperature rises through the high ambient range and towards the low engine temperature. However, when a fully warmed up engine is stopped, the thermostat must hold the choke valve slightly open, against the constantly applied biasing force ~- -that tends to swing it closed, for otherwise immediate re- --starting would be very difficult. Therefore the thermostat ~ -means of an automatic choke control device must not only be effective through one temperature range, to control choke , ','~h", 10~

opening, but must also be effective in another and substantially higher temperature range to prevent choke closing. -~
However, the torque force exerted by a coiled bimetal strip varies with temperature at a rate that is (for all practical purposes) uniform throughout its range of response.
Since the total rotation to be imparted to a choke valve, for swinging it between its fully open and its fully closed posi-tions, is about 90, the control member that is rotated by a choke control thermostat must be confined to rotation through -only a relatively limited angle. The control member must be j,~
brought to its high temperature limit of rotation before the engine is fully warmed up, and it must remain in that same ro- ~
tational position as the engine temperature continues to rise ~ -to the fully warmed-up value. Hence, once the control member has reached its high temperature limit of rotation, a single -coiled bimetal thermostat develops increasing stress with in- -~
creasing engine temperature.
If a single bimetal thermostatic element is to provide effective control of choke opening, and especially if -it is to afford control that will make for easy starting at very -low ambient temperatures, it must comprise a relatively long -strip that is coiled into relatively numerous convolutions, ~-and therefore the stresses developed in it when the engine comes - -up to its normal high operating temperature can be great enough ~
to deform it permanently. Once so deformed, the thermostat is - -of course no longer effective for cold weather starting. With the prior arrangement, the only alternative to risking such --deformation of the thermostat was to provide a thermostat that ; -inherently made for poor starting in very cold weather.
In the thermostatic control device of the present invention these disadvantages are overcome by the provision of two bimetal strips for positioning the control member, one of 1~ 3 said bimetal strips being a low temperature thermostatic ele-ment that is arranged to act upon the control member for positioning it when temperatures are in a low range and to be effectively disconnected from the control member at high tem-peratures, the other bimetal strip being a high temperature thermostatic element that is arranged to act upon the control member for positioning it when temperatures are in a high range and to be effectively disconnected from the control member at low temperatures.

Each of the bimetal strips responds to a tem- ~
perature change which is of opposite sense from that to which -the other responds; that is, one bimetal strip exerts increas-ing force between its opposite ends in response to increasing -temperatures, while the other exerts increasing force between its opposite ends in response to decreasing temperatures.
Further, each bimetal strip is in a unidirectional driving - ~ -connection between the control shaft and relatively fixed structure, so that each can act upon the control shaft in re- - -sponse to temperatures within its own range but will be effec- -tively disconnected from the control shaft at temperatures --in the range in which the other positions the control shaft.

Any thermostatically responsive automatic choke -control device must of course be adjusted to ensure that when a specified temperature prevails, it will effect a specified positioning of the member that it controls. With a thermo-static control element that comprised a single bimetal strip, factory adjustment was uæually accomplished in such a manner as to afford a specified positioning of the control member ~ -_ S _ lO~
at a specified low temperature, and the desired responses of the thermostatic element at high temperatures were obtained by maintaining close tolerances on the temperature response para-meters of the bimetal itself. With dual bimetal elements, as in the choke control device of the present invention, the temperature response parameters of the two bimetals need not be maintained within particularly close limits, provided that each of the bimetal elements is capable of individual and rela-tively facile adjustment. :Inasmuch as the two coiled bimetal strips of the thermostatic control device of this invention must act upon a single control shaft, they must be connected to the shaft at different points along its length, and since both strips must be seated in a well that is formed in the engine cylinder cast- r~.-ing, one of them must almost inevitably block access to the other. Thus it was by no means obvious how the mechanism could be arranged to afford individual, accurate and convenient ad-justment of both bimetal strips, particularly since this ob-jective had to be achieved in a structure that was simple, com- . . -pact, inexpensive and relatively resistant to tampering.
With the foregoing consideraLions in mind, it -:-i8 the general object of this invention to provide a thermo-static choke control device of the character described, fea- -turing dual bimetal elements, one for controlling the position :of a member in a high temperature range and the other for con- . : . -trolling the position of that member in a low temperature range, . :
wherein each of said elements is very readily adjustable in-dependently of the other so that each can establish said member in a predetermined position when a predetermined temperature pre-vails that is within the range to which the bimetal element ;: :
responds.
It is also an object of this invention to provide a thermo8tatic choke control device having dual bimetal elements , ~ , , , 41~5;~
and affording individual adjustability of them as just explained, which device comprises simple, compact and inexpensive structure that can be readily sealed to prevent tampering that might damage the relatively delicate bimetal elements or disturb factory established adjustments of them. . -~= .~. Thus the invention provides, in one aspect an internal combustion engine having relatively fixed structure ~ -comprising a carburetor with an air inlet duct, a choke valve -in said duct confined to opposite opening and closing movements, and a choke control shaft confined to rotation in opposite choke opening and choke closing directions and having a con-nection with said choke valve whereby the position of the choke valve is influenced by the position of said shaft, said engine being characterized by: a pair of spirally coiled bi-metal strips, each encircling said shaft with its inner end connected thereto, the inner ends of said strips being axially adjacent to one another and one of said bimetal strips being a high temperature thermostatic element that exerts an increas~
ing force between its end portions in response to increasing . .
temperature, the other being a low temperature thermostatic . -20 elemènt that exerts an increasing force between its end portions -~;-in response to decreasing temperature; means on said relatively fixed structure defining a well in which there is received the - :~portion of said shaft to which the inner ends of said bimetal --strips are connected; tele?scoped large and small diameter cylin-ders received in said well, each of said cylinders having one .
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of said coiled bimetal strips received therein and having in .:
its interior a circumferentially facing shoulder engageable by the outer end of its bimetal strip, said shoulders facing in . . -opposite circumferential directions; cooperating means on said relatively fixed ~tructure and on the large diameter cylinder _7_ ~1'~' .
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for confining the latter in any one of a plurality of different positions of rotational adjustment; and cooperating means on said cylinders for confining the small diameter cylinder in any one of a plurality of different positions of rotational adjustment.
.... , . . _ _ _ _ , _ The accompanying drawings illustrate one complete example of an embodiment of the invention constructed accord-ing to the best mode so far devised for the practical applica-tion of the principles thereof, and in which:
Figure 1 is a perspective view of a vertical crankshaft engine equipped with the automatic choke control of this invention, a part of the blower housing being shown broken away and the muffler and the air cleaner being omitted from the view;
Figure 2 is.a perspective view of the choke - -, control in its relation to adjacent portions of the engine : :
and carburetor;
. . .
Figure 3 is a side view of the engine cylinder -:
casting per se, with the choke control assembled thereon and ~ -the choke valve shown in broken lines; .. - -.--Figure 4 is a sectional view taken on the plane ::.
of the line 4-4 in Figure 3; -.
Figure S is a fragmentary top view of that portion of the cylinder casting on which the choke control is mounted;
Figure 6 i8 a fragmentary bottom view of that . :
portion of the cylinder casting on which the choke control is mounted;
; Figure 7 (which appears on sheet 6 of the draw-; 30 ings) i~ an exploded perspective view of the several parts ~;
of the control; ~.
Pigure~ 8 and 9 (on sheet 5) are diagrammatic views illustrating the manner in wh~ch the two oppositely ~ .
~ .

acting thermostats function to control choke valve position.
Figure 10 is a view generally similar to the lower right hand portion of Figure 4 but illustrating specifi-cally and in more detail the structure of that portion of the control device that permits the same to be adjusted to different requirements;
Figure 11 is a cross-sectional view through Figure 10 on the plane of the line 11-11; and Figure 12 is an exploded perspective view of parts of the structure shown in Figure 10.
Referring now to the drawings, the numeral S
designates the cylinder casting of a well-known single cylinder air cooled engine of the vertical crankshaft type. The crank- -shaft of the engine has a flywheel 6 mounted on its upper end portion. As is customary, impeller vanes 7 formed on the fly- -wheel induce a flow of cooling air through a blower housing 8 , and over the hot surfaces of the engine. -The cylinder casting has the usual intake and exhaust ports 9 and 10, respectively, the latter being threaded to provide for the attachment of a muffler (not shown) and the -former receiving the fuel mixture delivered to the engine by its -carburetor, which is generally designated by the numeral 11. -The carburetor has an elongated tubular body 12 with flanges 13 -- -at one end thereof which are bolted to the cylinder casting to ~-~
; , mount the carburetor. The opposite end of the tubular carbu-retor body is formed to provide an upwardly facing air inlet port 1~ onto which an air cleaner (not shown) is attached. The - -.. . . . .
interior of the tubular carburetor body, between the air inlet port and its discharge end, comprises a mixing passage in which there are the customary venturi and throttle valve, neither of which is shown, and between the throttle valve and the air in-let port is the choke valve 15 of the carburetor, which of course . .
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serves to regulate the admission of air into the mixing passage.
Fuel enters the mixing passage from a float bowl 16 at the underside of the tubular body that has a fuel inlet fitting 17 to which a fuel supply line (not shown) connects.
The choke valve 15 is of the butterfly type and - - ;
hence comprises a disc fixed to a shaft 18 that is journaled in coaxial bores in diametrically opposite side walls of the tubu-lar body, the axis of the shaft being horizontal. For impart-ing choke valve actuating rotation to the shaft 18, an actuating ;
lever 19 is fixed to its end adjacent to the cylinder casting, -~
and at its other end a weighted lever 20 is fixed to the shaft to bias the choke valve towards a defined closed position.
When the engine is running, the choke valve tends ~
~` to be opened by an actuator comprising an air vane 21, under -, the force exerted upon the air vane by the stream of engine cool- ~ -ing air flowing through the blower housing. The vane 21 is freely rotatably mounted on a vertical control shaft 22 that is in turn rotatably journaled in a bearing boss 23 on the cylin- - -' der casting. To enable the vane to swing freely on the control -`-, 20 shaft 22, the vane is part of a plastic molding which comprises a choke valve actuator and which has an elongated hub 24 wherein the upper portion of the shaft 22 is received. The vane 21 pro-jects radially from the upper portion of the hub 24. The choke valve actuator also comprises a radially projecting arm on the ~ -lower portion of the hub that has its extremity connected by means of a link 26 with the actuating lever 19 on the choke valve shaft 18. The choke valve and the air vane are thus directly linked, so that the choke valve is constrained to swing in cor-respondence with rotation of the air vane about the axis of the shaft 22.
It will be apparent that when the engine is not running and the bias provided by the weighted lever arm 20 is 1~4~ i3 not restrained, the choke valve will be in its defined closed position and the air vane will occupy a position extending across the path of the stream of engine cooling air that flows through the blower housing when the engine is in operation. -As best seen in Figures 4 and 7, there are abut- -ments 29 and 30 on the control shaft 22, directly above and beneath the boss 23, which cooperate with that boss to confine the shaft against axial displacement without interfering with its free rotation. The lower abutment 30 is provided by a C-washer snapped into a groove in the shaft directly below the underside of the boss, and the upper abutment 29 comprises a pair of diametrically opposite swaged ears projecting from the shaft and seated on a washer 31 that is interposed between those ears and the boss 23.
The control shaft 22 has a transverse slit 32, ~ -., :,. .
opening to its bottom end and somewhat elongated axially, to -~
provide for its attachment to a pair of spirally coiled bimetal strips 33 and 34. These bimetal coils comprise the thermo- - ~
static control means that governs the response of the choke ~ --valve to the opening force produced by the air vane actuator. - ---To perform their function, the thermostats must be so located --as to be sensitive to the heat of the engine in operation, -~
and to that end the thermostats and the lower slitted end por- Y~
tion of the shaft 22 are located in a cavity 35 formed in a ~ -3 lug 36 that is in close juxtaposition to the exhaust port 10 and is preferably formed integrally with the cylinder casting, - -~
as shown.
The cavity 35, with its cylindrical inserts that are described hereinafter, defines a two-diameter cylindrical well which open8 to the underside of the lug 36 and which is coaxial with the bore through the lug 23 that constrains the -shaft 22 to rotation. In the end wall of the cylindrical well :

there is a hole which is concentric with the cavity and through which the control shaft 22 projects into the cavity with a close but freely rotatable fit. The axially innermost small diameter upper portion 37 of the cylindrical well is axially shorter -than the large diameter lower portion 38 thereof. The bimetal coil 33, which is the smaller of the two and can be considered a hot thermostat, is received in the upper portion 37 of the cylindrical cavity; the cold thermostat 3~ is received in the larger diameter lower portion of the cavity.
The mouth of the cylindrical well or cavity 35 is closed by a Welsh plug 41, to preclude tampering with the bi-metal elements.
The inner convolutions of the bimetal coils have - ~ -radially in-turned end portions that are seated in the slit 32 in the control shaft 22, so that as the coils wind and unwind they can impart torque to the control shaft. ~ ;
~ As best seen in Figures 6, 8 and 9, each of the -~
; well portions is formed with a shoulder 39, 39' that projects radially inwardly from its cylindrical side wall. These shoul- `
l 20 ders provide stops against which radially out-turned end por-.~ , .. .
tions of the outer convolutions of the two spirally coiled bi- ~ -metal strips 33, 34 can abut and react as the coils wind and unwind in response to temperature changes. It is to be ob- -- -, served, however, that the shoulders 39, 39' provide unidirec-tional connections between the bimetal strips and the fixed - -structure comprising the lug 36, so that each of the thermo-stats can impose *or~ue; upon the control shaft 22 in only one direction of its rotation. The shoulders 39, 39' face in op-posite directions, and the two bimetal elements are oppositely coiled and also have their higher coefficients of expansion at - ~-oppo8ite face8, Hence, as explained hereinafter, the bimetal ii coils alternate with one another in controlling the rotational ~ orientation of the control shaft 22.
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To guard against interference between the coils of the superimposed bimetal elements, a washer 40 is inter-posed between them.
As noted hereinbefore, the choke valve is con-strained to move with the choke valve actuator that comprises the air vane 21 and its hub 24, but the choke valve actuator ; -is free to turn with respect to the shaft 22 to which the ther-mostats are connected. To enable the thermostats to limit opening of the choke valve in response to the opening force exerted upon the air vane by the stream of cooling air that im- -pinges it, there is a lost motion connection between the con-trol shaft 22 and the choke valve actuator. One element of --the lost motion connection comprises a finger 28 which is I formed integrally with the plastic molding and which projects ,~., ,,-., -;, .
s radially inwardly from the air vane across the top of its hub ~r 24, as best seen in Figure 7. The other element of the lost 3 m~tion connection comprises an angularly slotted collar 43 which is angularly adjustably fixed to the upper end portion of the control shaft 22 and which overlies the upper end of the hub '~
' 20 portion 24. f~n angular slot 44 in the peripheral portion of - ~ :the collar 43 accommodates the fin or finger 28. ~ - ;-As will be readily understood, the circumferen-~ tial length of the slot 44 determines the extent of rotary lost .~ motion between the air vane and the control shaft 22, and the angular position of the collar 43 on the shaft 22 determines ~,~; the relationship of the lost motion with respect to the effect of the thermostatic elements upon the shaft position.
The angle through which -the control shaft 22 can ` -, turn in response to the balance of the several torque producing forces exerted upon it by the air vane actuator, the weighted -~
arm 20 and the thermostatic elements is limited by a second circumferentially slotted collar 47, adjustably fixed to the ~, , .', -,~: , . . . .

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shaft 22 beneath the hub portion 24 of the air vane unit and just above the swaged ears 29. To define the limits of rota-tion of the control shaft 22, the ends of an arcuate slot 46 in the collar 47 collide with a stop fin 48 that projects from the top of the boss 23. -It will be apparent that rotational adjustment of the collar 47 on the control shaft 22 establishes the tem-perature range at which that shaft tends to be maintained at each of its limits of rotation. The adjustability of that col-lar thus enables the automatic choke control device of this invention to be readily adapted for installation on different types of carburetors, as for example updraft carburetors which require relatively heavy choking and horizontal draft car-buretors which require less choking. Rotational adjustment of the collar 43 on the control shaft enables the automatic choke device to be readily mated with any particular linkage between ~-a choke actuator and a choke valve. -- Since the hub portion 24 of the plastic mold- -ing is axially confined between the collars 43 and 47, both -of which are fixed to the shaft 22, those collars confine the plastic molding to rotation relative to the control shaft.
Returning now to a consideration of the thermo- -~, stats 33 and 34 and the manner in which they cooperate with one another, the bimetal strip comprising the cold thermostat is coiled in the direction to have its surface with the larger coefficient of expansion radially outermost so that the cold thermostat contracts and curls up with increasing temperatures.
The hot thermostat strip is coiled in the opposite direction ~-, so that upon heating it expands and unwinds. The shoulders 39, 39' face in opposite circumferential directions such that each shoulder take~ the reaction to unwinding or expansion of the bimetal coil with which it cooperates.
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1~41k~S3 As seen in Figures 8 and 9, the control shaft 22 rotates clockwise towards its high temperature range rota-tional position, at which it permits full opening of the choke valve.
When the engine is cold, the larger cold coil 33 reacts between its shoulder 39' and the control shaft to urge the control shaft towards its low temperature range posi-tion at which the air vane choke actuator is inhibited from opening the choke valve. At very cold ambient temperatures the ~
cold coil 33 exerts sufficient torque force upon the control ~ -shaft 22 to substantially prevent movement of the choke actua- -tor and thus hold the choke closed or nearly closed even when the engine is running. As temperatures increase from the very -j cold ambient level, the cold coil gradually contracts and winds ;, up, decreasing its torque force upon the control shaft and thus allowing the air vane actuator to take over and swing the choke valve towards its open position in step with the rising tem- -peratures. As the temperature rises above the highest level at ~ -~' which the choke valve should be restrained against fully open-ing, the cold coil curls or winds up enough to disengage itself ~, from its shoulder 39', so that the cold coil is not subjected ~ -.- to any externally imposed stress at such high temperatures. -Meanwhile, the hot coil 33 is functioning in a manner opposite to the operation of the cold coil, inasmuch as ~ -' it unwinds or tends to straighten out upon heating and exerts torque force upon the control shaft 22 only at high tempera-~; tures. Thus, when the engine is hot, the hot coil holds the 3~ control shaft 22 at its high temperature position of rotation.
The choke valve is then held slightly open when the engine is stopped and can be fully opened by the air vane actuator as soon as the engine begins to run. At lower temperatures the cold coil controls the rotational position of the shaft 22, and J :
, - 15 -the hot coil is disengaged from its shoulder 39 so as not to be subjected to any externally imposed stress.

It will be apparent that each bimetal coil is ,~' subjected to only a limited externally imposed stress, not high enough to permanently deform it, owing to the fact that each coil exerts torque upon the control shaft 22 through only a relatively narrow range of temperatures.
As will be evident from what has been said above, - -the circumferential locations of the shoulders 39 and 39' in -the cavity 35 have a significant bearing upon the results ob-tained with the control device, inasmuch as they cooperate ~ -with the out-turned end portions of the outer convolutions of -the respective spirally coiled bimetal thermostats 33 and 34 to provide oppositely facing unidirectional force resisting connections between the thermostats and the fixed structure. -If tolerances on the bimetal thermostats are closely main- ~ :-.~ ... ..
j tained, the circumferential locations of those shoulders can j be predetermined and then established in the design of the lug ;~
-36 that defines the cavity 35. However, the thermostats can -be manufactured less expensively and other obvious advantages can be obtained by providing for the adjustability of the locations of those shoulders in accordance with the principles of this invention.

As shown in Figures 10, 11 and 12, the lug 36 has a cylindrical well S0 therein to receive a pair of tele-scoped cylinders Sl and 52, the former being of size to fit --the well 50 and being larger in diameter and in axial length than the cylinder 52.

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The bore of the larger cylinder 51 is stepped to provide an inner small diameter bore portion 53 in which the high temperature bimetal element 33 is located, and a larger diameter bore portion 54 in which the cylinder 52 is received.
The low temperature thermostatic element 34 is located in the bore of the smaller cylinder 52.
The shoulder 39 against which the high temperature bimetal reacts is formed in the small diameter bore portion of the cylinder 51. The shoulder 39' against which the low tem-perature bimetal reacts is formed in the bore of the smaller cylinder 52.
The contiguous surfaces of the cylinders 51 and 52 are formed with interengaging axially separable serrations ~ -55, so that the two cylinders can be brought into telescoping relationship to one another with their respective shoulders 39 and 39' in any desired positions of relative angular adjustment.
In like manner, the contiguous cylindrical surfaces of the larger cylinder and of the well 50 are formed with axially sepa-rable interengaging serrations 56 whereby the shoulder 39 can be disposed in any desired angular position relative to the engine body. Obviously, the splined connections between the respective cylinders, and between the outer cylinder 51 and the lug 36, constitute only one form of rotatably adjustable con-nection between these elements, and other types of connections suitable for the purpose will suggest themselves.
In assembling the device, the larger cylinder 51 -: . .
is first inserted into the well and is rotatably adjusted as necessary to bring its shoulder 39 to the angular position at which a predetermined high temperature response is obtained for the high temperature bimetal element received in its smaller diameter bore portion. Thereafter the smaller cylinder and -the low temperature bimetal element are installed and similarly adju8ted for a predetermined low temperature response. The ii3 washer 40 is of course interposed between the contiguous axially facing surfaces of the two cylinders and hence between the two bimetal elements. The Welsh plug 41 holds the parts assembled and blocks access to the cylinders and bimetal elements to pre-vent tampering.
From the foregoing description taken with the ; accompanying drawings it will be apparent that this invention provides a thermostatically governed choke control device that is especially well adapted for small engines because of its simple, compact, tamper-proof construction and its capability for reliably affording choke control under a wide range of -ambient temperatures. -~ Those skilled in the art will appreciate that : the invention can be embodied in forms other than as herein dis- ~
closed for purposes of illustration. -.. ' :
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Claims (2)

The embodiment of the invention in which an exclusive property or privilege is claimed is defined as follows:

1. An internal combustion engine having relatively fixed structure comprising a carburetor with an air inlet duct, a choke valve in said duct confined to opposite opening and closing movements, and a choke control shaft confined to rota-tion in opposite choke opening and choke closing directions and having a connection with said choke valve whereby the position of the choke valve is influenced by the position of said shaft, said engine being characterized by: a pair of spirally coiled bimetal strips, each encircling said shaft with its inner end connected thereto, the inner ends of said strips being axially adjacent to one another and one of said bimetal strips being a high temperature thermostatic element that exerts an increasing force between its end portions in response to increasing tem-perature, the other being a low temperature thermostatic ele-ment that exerts an increasing force between its end portions in response to decreasing temperature; means on said relatively fixed structure defining a well in which there is received the portion of said shaft to which the inner ends of said bimetal strips are connected; telescoped large and small diameter cylin-ders received in said well, each of said cylinders having one of said coiled bimetal strips received therein and having in its interior a circumferentially facing shoulder engageable by the outer end of its bimetal strip, said shoulders facing in opposite circumferential directions; cooperating means on said relatively fixed structure and on the large diameter cylinder for confining the latter in any one of a plurality of different positions of rotational adjustment, and cooperating means on said cylinders for confining the small diameter cylinder in any one of a plurality of different positions of rotational adjust-ment.

2. The internal combustion engine of claim 1 wherein said cooperating means on the relatively fixed structure and on the large diameter cylinder comprises interengageable axially extending serrations on the outer cylindrical surface of the large diameter cylinder and on its contiguous surface of said well; and wherein said cooperating means on said cylinders comprises interengaging axially extending serrations on the contiguous cylindrical surfaces of said cylinders.