CA1043202A - Carburetor and fuel supply system - Google Patents

Abstract

ABSTRACT OF DISCLOSURE
A carburetor and fuel feed device for small internal combustion engines incorporating a pumping section which re-ceives fuel from a supply and delivers the required amounts for the range of operation between high and low speed operation with any excess being returned to the source of supply. A
control device is employed which permits deceleration from high to low speed operation without flooding by instantaneously preventing the supply of fuel for high speed operation in re-sponse to movement of the the throttle valve to an idle or low speed position.

Description

A-308 Phelps ~3Z(~2 BACKGROUND OF THE ~NVENTION
. . . _ . _ This invention relates to a combined carburetor and fuel feed device for small internal combustion engines.
In the operation of small internal combustion engines which are used with power tools such as chain saws and the like, the carburetor must be very small and compact and operate in -all positions of the power tool and at the same time function in a sophisticated manner just as larger, full sized carbure-tors for larger internal combustion engines.
Conventionally, such carburetors and fuel feed systems employ a diaphragm pump which delivers fuel to a metering , - . .
chamber which also includes a diaphragm. The metering chamber meters fuel to both a high speed circuit and a low speed cir~
cuit from which fuel is fed to a mixing chamber. The diaphragm fuel pump is actuated in response to crankcase pressures which fluctuate between sub-atmospheric and super-atmospheric ;
pressures. As a consequence, the speed of the engine determines the amount o fuel flow which i5 supplied by the pump. Since the speed of the engine varies between idle speeds of approxi~
mately three-thousand rpm's to high speed, to the order of eight-thou~and rpm's or more, a large amount of fuel mu~t be -supplied for high speed operation. The metering chamber must be very sensitive to meter the correct amount of fuel to the mixing chamber of the carburetor. Usually the metering chamber employs a diaphragm which reacts to the pressure of fuel at one side to move and operate a fuel inlet valve. The dia-phr~gm must be very sensitive to changes in pressure and must operate in all positions of the system to meter the appropriate amount of fuel. Such devices are difficult to control partic-ularly at low speeds when low volumes of fluid are being delivered. The problem becomes particularly acute when the ~ -~ .
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engine is operating at high speed and it is desired to reach -an idle speed as rapidly as possible. Such a condition occurs, for examplé, when a chain saw is being operated under load conditions and a saw cut is completed. At that time the load on the engine is relieved and the engine begins to operate at a maximum speed. The operator under such conditions will move the throttle to a closed or idle position. Unless the metering chamber can react very rapidly, fuel continues to be fed to the high speed circuit so that even though the throttle is closed, excess fuel is being fed to the mixing chamber which causes a flooding resulting in stalling of the engine. The engine must then be restarted and because o~ the ;
flocded condition starting usually is difficult. ~ -SUMMAR~ OF TH~ INVENTION
- .. : , It is ~he object of the invention to provide a small compact carburetor and fuel feed system for use with power tools such as chain saws in which provision is made for instantaneous deceleratlon of the engine from high speed to low speed without flooding the en~ine.
Still another object of the invention is to provide a carburetor and fuel supply apparatus which incorporates high and low speed fuel delivery circuits and in which the high speed circuit will be instantaneously closed when the throttle is closed for low speed operation.
It is another object of the present invention to provide a combined carburetor and fuel supply system which eIiminates the need for a diaphragm type fuel metering chamber and in which fuel is delivered to the degree required for the ,. . .
speed of operation of the engine and any excess is returned to the fuel tank.

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Another object of the invention is to provide a com-bined carburetor and fuel supply system in which closing of the carburetor throttle valve when the engine is operating at high speed, closes off any further fuel delivery ~o the high speed circuit to prevent flooding of the carburetor and re- -sults in an immedia~e return of the engine to idle speed.
The co~bined carburetor and fuel supply system of the present invention incorporates a diaphragm operated pump which pumps fuel from a fuel supply tank back to the fuel supply tank and in which the high speed circuit and low speed circuit of a carburetor extract the required amount of fuel from the fuel line between the pump and the tank with an~ -~
excess fuel not required by the engine being returned direct- i~`
ly to the fuel tank. The carburetor also includes a control mechanism in the high speed circuit which is operative in all positions of the carburetor to close off the supply of fuel to the high speed circuit whenever the operator signals a demand to drop the engines speed from full speed to idle qpeed. The control operates to cut of the fuel supply so that ~looding and stalling of the engine is avoided.
D~SCRIPTION OF THE DRAWINGS
Figure 1 is a diagramatic view of the carburetor and fuel feed apparatus embodying the present invention;
Figure 2 is a view taken generally on line 2-2 in Figure l; and Figure 3 i5 a diagramatic view showing another embodiment of the invention.
DETAILED DESCRIPTION ,;
The carburetor and fuel supply system embodying the present invention is incorpora~ted in a single body portion ~-~

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10 which in the drawings is depicted by separate housing por-tions. Incorporated within the body portion 10 are a fuel pump 12 and a carburetor section 14 by which fuel is supplied to the carburetor to be mixed with air and delivered to the crank-case of an engine such as a two cycle engine, for example:
In two cycle engines, air and fuel is drawn through the carburetor into the crankcase of the engine and during the -compression stroke, the mixture of air and fuel is bled from ;
the engine crankcase into the engine cvlinder. The pressure of gases within the crankcase of ~he engine undergoes a change from sub-atmospheric or vacuum pressure to pressure above atmospheric. These pressure fluctuations are utilized for operating the pump section 12 of the combined carhuretor and fuel supply system of the present invention.
The carburetor Section 14 of the apparatus includes a stepped bore in a form of a relatively large diameter bore 15, an intermediate bore 16 and a small bore 17 which merge with each other. The bores 15 and 17 receive a venturi fit-ting 18, which is generally tubular in configuration. The interior of the fitting 18 forms a passage which has a relative-l~v large opening 19 at one end merging with a restricted pass-age or venturi portion indicated at 20. The lower portion of the fitting 18, as viewed in the drawings protrudes radially from the walls of the bore 17 and is undercut as indicated at 21 to form a groove facing axially toward one end of the bore 17. The fitting 18 is held in position in the bore portions `
15 and 16 by means of a fuel nozzle 22 which is threaded into the walls of the housing 10 to project through a radial open-ing 23 in the fitting 18. The nozzle 22 prevents accidental removal of the venturi fitting 18 from the housing 10. The `
venturi fitting 18 and the bore portion 17 in the housing

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10 form a fuel and air mixing passage 24 by which air enters at the opening 19 and is delivered to the open end of the bore 17 which communicates with the intake manifold of the engine.
The venturi portion 20 serves to create a low pressure effect ~
during air flow due to the restriction of the venturi portion, ~ ;i and the undercut portion 21 tends to restrict the back flow of a mixture of air and fuel against the stream of incoming air, particularly at high operational speeds of the engine.
The venturi fitting 18 can be removed and replaced by another venturi fitting having different interior con-figuration and different sized venturi portion 20 to accommo- ~ :
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date different types of engines with different types of oper-ating characteristiss without requiring changes in the remain-ing portion o~ the carburetor.
Mounted within the relatively large opening 19 o `
the venturi fitting 18 at the upstream side of the venturi ~ ~
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fitting 20 is a conventional choke valve 24 of the butterfly type which is mounted for rotation with a shaft 26. The shaft 26 is rotatable in the walls of the venturi fittings 18 `
and hou5ing portion 10 adjacent the carburetor section 14.
At the opposite, or down stream side of the venturi restriction 20, and in the ~mall bore portion 17, is a conventional butter-fly type throttle valve 28 which is mounted on a shaft 30 rotatably journaled in the walls of the housing 10.
Fuel for operating the engine is stored in a tank 32 and is pumped therefrom through a line 34 to the pump 12 : ~
which delivers the fuel to a line 38 returning to the tank 32. ~ ;
Disposed within the fuel tank 32 is a fuel inlet t~be 40, the free end o~ which is provided with a fuel filter 42. The fuel . . .
inlet 40 is flexible so that for any position on the tank 32

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A-308 Phelps 1043Z02 the filter 42 remains immersed in fuel so that fuel can be de-livered through the hose 40 to the line 34.
A branch line 44 is connected with the fuel return line 38 and is delivPred through a control mechanism 46 through a line ~8 to a main or high speed fuel metering system indicated generally at 50. ~ second branch line 52 is connect-ed with the return line 38 for the delivery of fuel from the pump 12 to a low or idle speed metering system indicated gen-erally at 54.
The pump 12 which receives fuel from the tank 32 through line 34 anddelivers fuel through the line 38 back to -the tank 32, to the high speed metering system 50 or the low speed metering system 54 is of the diaphragm type. The pump 12 includes a pair of housing portions 56 and 58 forming part of the housing lO. A diaphragm 60 is sandwiched between the housing portion 56 and 58 and a portion of the diaphragm is located in a cavity ormed in the housing sections 56 and 58 and serves to divide the cavity into a fuel chamber 62 and a pumping chamber 64. The pumping chamber is connected by way Of a conduit indicated at 66 to the crankcase of the two ~-~
cycle engine so that the pressure pulsations created within the crankcase are also created in the pumping chamber 64 causing the diaphragm 60 to flex and to pulse. The member forming the diaphragm 60 also forms an inlet valve 68 and an outlet valve 70. The inlet valve 68 and outlet valve 70 are of the flap type and act as check valves. The flap valves 68 and 70 as well as the diaphragm 60 are formed of a single, unitary sheet member abricated o an elastimeric or rubber-.; . .
like material.
The inlet valve 68 controls the entry of fuel to the

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pump 12 from the line 34 and the outlet valve 70 controls the ::
delivery from the pump 12 to the return line 38. Upon the .
pulsation of pressure in the pumping chamber 64, the diaphragm .
60 flexes to draw fuel from the line 34 through the valve 68 and upon flexing of the diaphragm in the opposite direction fuel is delivered through the valve 70 to the return line 38.
For example, when sub-atmospheric pressure exists in the pump- . -ing chamber 64, the diaphragm 60 will move downwardly from ; .-the position shown in the drawing causing a low pressure in the .~ -fuel chamber 62 this causes the flap valve 70 to close and ~ :
the flap valve 68 to open and to draw fuel from the tank 32 through the line 34 into the chamber 62. When the pressure in the pumping chamber 64 changes to super-atmospheric pressure, the diaphragm 60 will flex upwardl~ increasing the pressure in lS the fuel chamber which tends to close the flap valve 68 to :
prevent fuel return through the line 34 and at the same time the increased pressure opens the flap valve 70 so that fuel is delivered from the chamber 62 to the line 38 and to its connect~ .
ing branch lines 44 and 52.
Fuel which is delivered b~ the pump 12 ~o the outlet or return line 38 is also made available in the branch lines 44 : . .:
and 52. The first o the branch lines 44 is connected through .. .
the control mechanism 46 to the line 48 and to the high speed metering circuit 50. The high speed metering circuit S0 in- `;
cludes a needle valve 72 which is threaded in the housing 10 so that it may be threadably adjusted to selected positions to vary the effective size of a passage 74 in the nozzle 22 commurlicating with the mixing passage 24 at the venturi re~
striction 20. Ad~ustment of the high speed needle valve 72 . ~. .',;"'-' ':.

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controls the amount of fuel delivered during high speed oper-ation of the engine.
The second of the branch lines 52 is connected to the low speed metering portion 54 which includes a needle valve 76 which like the high speed needle valve 72 is threaded in the housing 10 and which may be threadably adjusted to vary the orifice communicating with a fuel manifold portion indi-cated at 78. The manifold 78 includes a plurality of passages 80, 81 and 82. In the closed position of the throttle valve 28 shown in the drawings, passage 82 opens downstream of the throttle 28 and the passages 80 and 81 communicate upstream of the closed throttle valve. Under such conditions the lower passage 82 draws fuel from the manifold 78 due to the high vacuum existing with the closed throttle valve 28. The other two passages 80 and 81 serve to supply the air require-ments for mixiny the fuel entering through the passage 80 so that a combustible mixture is formed for delivery to the in~
take manifold o the engine.
The control mechanism 46 disposed in the high speed uel delivery lines 44 and 48 includes a flexible diaphragm 84 which is sandwiched between housing sections 86 and 87 and divides a cavity into a chamber 88 and a signal chamber 90. A
stem 92 extends from a central portion of the diaphragm 84 and is connected thereto in any conventional manner as by a . : .
rivet 95. ThP lower end of the stem 92 projects into a cavity 94 formed in the housing section 87. As best seen in .. .. .
Figure 2, the stem 92 is provided with a transverse opening 96 which loosely receives one end of a lever 98 which is dis-posed in a bore 100 communicating with the cavity 94.

An intermediate portion of the lever 98 is provided

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with an O-ring 102 which sealingly engages the lever 9B as well as the bore 100. The opposite ends of the lever 98 move upwardly and downwardly from the position in which the lever is shown in the drawings in response to flexing of the diaphragm ~
84 which acts as an actuating means causing the lever g8 to -pivot about a fulcrum which is afforded by the O-ring 102.
The free end of the lever 98 moves relative to a ball valve 104 which serves to open and close relative to a -conical seat 10~ formed in the lower housing section 87 and communicating with the fuel line 48. The lower housing ;
section 87 also is provided with a fuel inlet port 108 which communicates with the branch fuel line 44 and with the bore 100. Fuel which is delivered through the branch line 44 passes through the ball valve 104 which is maintained in an open position by a small spring 109, to the delivery line 48, and `
to the high speed metering portion of the carburetor 14.
When the ball valve 104 is closed on its seat 106 under the in-1uence of the action o the lever 98, the supply of fuel i~ terminated.
The signal chamber 90 of the control mechanism 46 communicates b~ way of a conduit 110 to a passage 112 which opens into the mixing passa~e 24 downstream of throttle valve 28~ The chamber 88 formed at the opposite side of the dia-phragm 84 is in continuous communication with the atmosphere.
~tmosphere pressure also will exist in the various cavities .. . .
surrounding the stem 92 and in the bore 100 at the right side of the O-ring 102 as seen in the drawing. Fuel entering .:
through the line 44 and passing to the line 48 will exist in .: ,.
the cavities formed to the left of the O~ring 102. The O-ring serves as a seal separating the f~lel at one side from atmos-_g_ ,, .'' ' .. ~. . .

A-308 Phelps ~32~
pheric air at the other side and in addition serves as a pivot or fulcrum for the lever 98.
The small spring 109 which is disposed between the ball 104 and the conical seat 106 insures that the ball 104 will become unseated to permit the passage of fuel when the lever 98 has been pivoted in a clockwise direction as viewed in the drawing. During usual operations of the carburetor, the free end of lever 98 is spaced suficiently from the valve seat 106 so that the ball 100 ma~ remain in an open position to permit full delivery of fuel from the line 44 to the line 48. : -Referring now to the embodiment of the invention:.
shown in Figure 3, a control mechanism 46' is employed in~
stead of the control mechanism 46 shown in the arrangement in lS Figure 1. The control mechanism 46' is disposed in the housing 10 to receive fuel from the line 44 and to deliver fuel to the ... -.
line 48 to the hiyh speed metering system 50. The control . ;~; .
mechanism 46' includes a ball valve 104' which is urged to an ~ :
open position by a ~qpring 109'. The ball valve 104' is moved to a closed position on a qeat 106' by mean~ of a lever 98'.
The lever 98' is adapted to pivot about an O-ring 102'. The : :
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lever 98' is disposed in an elongated chamber 120 and the O-ring . :
102' has its outer diameter in sealing engagement with the walls -of the chamber 120 and the inner diameter of the O-ring is in . ~ . .
engagement with the lever 98'. The O-ring 102' not only acts : . .
as a pivot for the lever 98' but also acts to seal the chamber . ~
120 below the O-ring as viewed in Figure 3 from the chamber ~ .
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122 above the O-ring 102'. .~:

The lower end of the lever 98' as viewed in F~gure 3, is provided with a cam arm 124 which projects into the ~ .

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small bore portion 17 of the fuel and air mixing passage 23.
The cam arm 124 is provided with a cam surface 126 which is adapted to be engaged by the throttle valve 28 when the latter ~:
is in its closed position, as shown in Figure 3, to act as an actuating means and pivot the lever 98' in a clockwise direction relative to the pivo~ formed b~ the O-ring 102'. It will be understood that the cam arm 124 is relatively short and that the passage 120 is disposed in close proximity to the fuel air mixing passage 23. However, in the interest of clarity passage 120 has been shown displaced a substantial distance to one side of the fuel air mixing passage 23.
In the open position of the throttle valve 2B, the . .
latter is out of engagement with cam surface 126 and the spring ~-109' is of sufficient strength to act on the ball valve 104' ..
and on the upper end of the lever 98' to pivot the latter in a ~ :
counter-clockwi~. dire~tions permitting the valve 104' to ~:. . -unseat from the conical seat 106' and permit the delivery of `: ;
fuel from the line 44 to line 48 and to the high speed meter-ing circuit 50. .
OPERATION ;~ . .
Referring to the embodiment of the invention shown in Figures 1 and 2, with the choke valve 25 in an open position, ~
movement of the throttle valve 28 to an open position will : ~:
cause air to flow rapidly in the passage 24 in response to ;~ ~.
vacuum at the engine intake manifold. At the venturi restric-tion 20, the airflow will provide a low pressure region so .
that fuel in the line 48, which is under a relatively higher pressure, will be delivered through the high speed fuel passage 74. The optimum amount of fuel for high speed engine operation is obtained by ad~ustment of the needle valve 72.

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When the engine is being operated at lower speeds with tha throttle fully or partial~y closed the pressurized fuel in line 52 will be delivered by way of one of the pass-ages 80, 81 and 82 of the manifold 78 due to the low pressure area in the fluid passage 16. Under either conidition, that is high or low speed operation, any fuel which is not pulled into the line ~8 or line 52 is returned to ~he tank 32 b~ way of the line 38 so that the high speed circuit 50 and the low speed metering circuit 54 are supplied only with the required amount of fuel and the remainder, if any, is returned to the tank 32.
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When the engine is being operated at full speed the choke 25 and throttle 28 will be in their fully open! position permitting the maximum passage of air and fuel to the engine lS manifold. This is a condition that could exist during a saw cut. Under these circumstances if the throttle valve 28 is suddenly closed, for example, upon completion of the saw cut, a maximum vacuum condition will be created at the downstream . . . . .
side o the throttle valve 28. This vacuum pressure is com-municated through the line 110 to the signal chamber 90 associated with the diaphragm 84 in the control mechanism 46.
This causes a differential of atmospheric pressure at the under- ;
side o the diaphragm 84 and a vacuum pressure at the opposite side of the diaphragm which causes it to move upwardly from .: . . .
the position seen in the drawings, carrying the stem 92 up-wardly. Upward movement of the stem 92 causes the lever 98 to pivot about the 0-ring 102 in a counter-clockwise direction as seen in the drawings to bring the free end of the lever 98 downwardly and into engagement with the ball valve 104. The ;-ball valve 104 is moved downwardly against the action of the ~
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light spring 109 to seat on the conical seat 106 and to close off the delivery of fuel to the line 48.
With the ball valve 104 closed, fuel cannot be de~ . .
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llvered to the high speed metering circuit 50~ However, fuel .
continues to be delivered through the branch line 52 to the : .
low speed metering circuit 54 for operation of the engine at a low or idle speed. By instantly closing off the fuel supply line 44, 48 to the high speed metering circuit 50, when the : : -throttle valve is closed during high speed operation no addi-tional means are required to dry the fuel mixing chamber 24 and flooding when decelerating from high speed to low speed is :
eliminated. Moreover, in any position of the combined car~
buretor and fuel delivery device for example, in a completely inverted position, the controlled diaphragm 84 operates to close the ball valve 104. The diaphragm 84 can be of a rela~
: : , tiv~ly small ~ize since the effect of differential pressure ~-:j . :
acting thereon is being magnified mechanically through the .`~ ~.
lever 98 so that a relatively large force is applied to the ball valve 104 to maintain it in a closed po~ition on its .
20 seat 106.
Referring now to the embodiment o the invention shown in Figure 3, the operation is generally imilar to that :
shown in Figure 1, in that the ball valve 104' is controlled to permit the delivery of fuel from the line 44 to the line 25 48 or to prevent such delivery when the ball valve 104 is closed on its seat 106'. ~ .
During normal operation of the carburetor with the .
throttle valve 28 in an open position, the spring 109' serves to move the ball valve 104l from its seat 106' to permit open 30 communication between the line 44 to the line 48 for delivery ~
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of fuel to the high speed metering circuit 50.
When the throttle valve 28 is moved to a closed position, the edge of the throttle valve 28 engages the cam surface 126 to pivot the arm 98' in a clockw,isë- direction bringing the upper end of the lever 98' into engagement with the ball valve 104'. Such movement of the lever 98' closes the ball 104' on the conical seat 106' against the action of the spring 109' and prevents further delivery of fuel from the ~ -line 44 to the line 48. With the delivery of fuel to the high -speed circuit 50 interrupted, fuel continues to be delivered through the branch line 52 and the low speed metering circuit 54 for operation of the engine at a low or idle speed. The instant closing off of the fuel supply line 44, 48 to the high speed metering circuit 50, when the throttle valve is closed, prevents flooding particularly when decelerating the engine from a high to a low speed. Furthermore, in any posi-tion of the carburetor and fuel delivery device, the ball valve 104' is positively closed upon movement of the throttle valve 28 to its clo~ed position.
It will be noted that in the em~odiment of the in-vention shown in Figure 3, closing of the throttle valve is effective to close the fuel delivery line 44, 48 at all times whether or not the engine is operating.
A carburetor and fuel supply system have been pro~
vided~in which an engine operating at high speed requiring a high volume of fluid may be almost instantaneously decel-erated to a low, idle speed at which only a low volume of fuel is re~uired. The rapid deceleration from high to low ~
speed is accomplished without the usual flooding condition by -;
employing a control which instantly and completely closes off :. .: . :
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fuel being delivered to the high speed circuit of the car-buretor in response to closing of the carburetor throttle valve to its low or idle speed position, after which fuel is ~- ;
delivered solely to the low speed circuit. Such operation oc- -curs for all positions of the carburetor. The carhuretor and fuel supply system is such that the full range of fuel vol~
ume required between high and low speed operations are accom-modated by a carburetor and fuel supply system which receives ;
fuel from a supply and delivers the required amounts to a high speed circuit of a carburetor section and to the low speed section with any7excess fuel not required by the carburetor -section being returned directly to the supply of fuel. The ~ ~ -carburetor employs a replaceable venturi section by which different performance characteristics, may be obtained to ac-commodate different engines with different operating character-istics and in which the venturi section incorporates an under-cut portion whi~h tends to restrict the back flow of fuel air mixture in the mixing chamber during high speed operation.

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Claims (18)

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. In a carburetor for operating an engine, a housing, a mixing passage formed in said housing for supplying a mix-ture of fuel and air to the engine, a throttle valve in said passage movable between open and closed positions for control-ling the supply of air and fuel mixture to the engine, a source of fuel supply, an idle metering system communicating said source of fuel supply to the mixing passage downstream from said throttle valve for operating the engine in the idle speed range, a main metering system communicating said source of fuel supply to the mixing passage upstream from said throttle valve for operating the engine in a higher speed range, valve means in said main metering system movable to open and close said source of fuel supply to said main meter-ing system, and control means operable upon movement of said throttle valve to a closed position to move said valve means to said closed position for operation of the engine in the idle .
speed range.

2. The combination of Claim 1 in which said control means includes a lever pivotally mounted in said housing and having an end portion engagable with said valve means to move said valve means to said closed position, and actuating means for moving said lever upon movement of said throttle valve to said closed position.

3. The combination of Claim 2 in which said actuating means includes a diaphragm disposed in said housing and form-ing a pair of chambers at opposite sides thereof, and means communicating one of said chambers with said mixing passage downstream of said throttle valve.

4. The combination of Claim 2 in which said act-uating means includes cam means engagable and movable by said throttle valve upon movement of the latter to its closed position to close said valve means.

5. The combination of Claim 2 and further comprising passage means communicating with said valve means, said lever being disposed in said passage for pivotal movement therein.

6. The combination of Claim 5 and further compris-ing a pivot element for said lever, said pivot element being in the form of a seal engagable with the walls of said passage and with said lever for preventing fluid flow through said passage means.

7. The combination of Claim 6 in which said seal is an O-ring mounted on said lever.

8. In a carburetor for operating an engine, a hous-ing, a mixing passage formed in said housing for supplying a mixture of fuel and air to the engine, a throttle valve in said passage for controlling the supply of air and fuel mixture to the engine, a pump having an outlet communicating with a return line for returning fuel to a source of fuel supply, an idle metering system communicating with said return line to supply fuel to the mixing passage downstream from the throttle valve for operating the engine in the idle speed range, a main metering system communicating with said return line for operating the engine in a higher speed range, valve means in said main metering system movable between first and second positions to open and close, respectively, the supply of fuel to said mixing passage, and control means operable up-on movement of said throttle valve to its closed position to move said valve means to said second position.

9. The combination of Claim 8 in which said con-trol means includes a diaphragm forming a pair of chambers in said housing, one of such chambers communicating with said mixing passage downstream of said throttle valve and the other of said chambers communicating with the atmosphere.

10. The combination of Claim 8 in which said valve means is normally disposed in an open position and in which said control means is operable to move said valve means to a closed position.

11. The combination of Claim 8 and further compris-ing lever means operable to move said valve means to said second position, said lever means being connected to said control means for movement thereby.

12. The combination of Claim 11 in which said lever means has one end connected to said control means and the other end is engagable with said valve means.

13. The combination of Claim 12 in which said control means is a cam mounted on said lever and having a portion dis-posed in said mixing passage for engagement with said throttle valve when the latter is moved to a closed position.

14. The combination of Claim 11 in which said lever means is disposed in an elongated passage communicating with said valve means, and a seal means disposed in sealing engage-ment on said lever and in sealing engagement with said elong-ated passage to prevent fuel passage from said valve means through said elongated passage and to form a pivot for said lever.

15. The combination of Claim 8 in which said main metering system is connected to said return line between said pump and said fuel supply and in which said idle metering system is connected to said return line between said main metering system and said source of fuel supply.

16. The combination of Claim 8 in which said fuel mixing passage includes a venturi portion restricting the flow of air in said mixing passage and in which said main metering system communicates with said mixing passage adjacent to said venturi.

17. The combination of Claim 16 in which said venturi includes a recess portion downstream of said venturi to re-strict backflow of fuel towards said venturi.

18. The combination of Claim 16 in which said venturi is detachably supported in said fuel mixing passage.