CA1059391A - Variable venturi downdraft carburetor - Google Patents

Abstract

TITLE: VARIABLE VENTURI DOWNDRAFT CARBURETOR

Abstract of the Disclosure A variable venturi downdraft carburetor comprised of an open car-buretor body enclosing an interior first venturi member and coaxial peripheral second venturi member. The second venturi member is integral with the carbur-etor body and defines a peripheral annular fuel dispensing orifice. The first venturi member is movable relative to the second venturi member along an upright axis. The venturi members together define an annular restricted air passage adjacent the fuel dispensing orifice that may be varied in size to increase or decrease the volume and velocity of air passing by the orifice.
Fuel is supplied from an annular fuel chamber to the orifice in response to movement of air thereby. Various air bleed provisions are made to insure proper air-fuel mixing throughout the entire operational range of the engine on which the carburetor is attached. Provisions are also made to bleed air into the fuel supply as it is being delivered to the mixing chamber in order to premix a charge of air and fuel before it enters the carburetor mixing chamber.

Description

)S~353~ : ~
" .
The present invention relates basically to the field of carburetion for internal combustion engines and more particularly to carburetors utiliz- `ing a variable venturi.
Most present automobile carburetors are designed with a fixed venturi to create a vacuum pressure in the induction duct in order to pull fuel from a fuel reservo~r. The venturi, by its fixed nature, operates at a maximum efficiency over a small range of engine R.P.M. Furthermore, a aifferent size venturi is generally required for different size engines in performance requirements. Seldom is an automobile operated in only the small range of speeds required for the maximum operational efficiency of the fixed venturi. Consequently, various complicated venturi arrangements including multiple venturi tubes have been included in carburetors in an attempt to make the carburetor more eficient over a wide range of operating conditions.
i . . . .
A venturi is generally mos~ ineffective at slow speeds in which the velocity of air through the induction duct is minimal. Special idle jets have been ; ;
incorporated in carburetors in order to overcome this problem by passing additional fuel to the engine at low R.P.M. At higher R.P.M., the fuel from the special idle jet is not required and therefore provides a richer mixture than is needed for that particular R.P.M. range. This results in loss of fuel in an over rich mixture of fuel to air. This loss is particularly notable ;
at high speeds and during deceleration.
Other features have been added to carburetors to overcome the limitations noted with the fixed venturi type carburetor. One limitation is recognized when the carburetor is operating at low engine R.P.M. When the operator desire~ to quickly accelerate the automobile, quick depression of ~the throttle pedal causes the throttle valve to rapidly open. This allows for ~ -additional amounts of air to flow through the venturi before the vacuum pressure is increased to a higher R.P.M. in order to draw additional fuel.
Consequently, it is not infrequent that the engine will stall on the initial lean mixture. To overcome this limitation, accelerator fuel pumps have been incorporated in carburetors to operate in response to depression o:~ the throttle pedal to pump additional fuel into the mixing chamber when the .. . , . . ~ ,. .

~(3S~93~

the throttle is depressed rapidly. However, such a compromise results in a ;~
loss of fuel when the vehicle is traveling at high speeds and the accelerator pump is temporarily released and then depressed again. The pump adds the additional fuel to the engine which is wasted since the high air velocity through the venturi is sufficient to pull enough fuel into the cylinders to ~-accomplish the objective without the necessity of the fuel pump.
It may be seen then th~t additional features added to existing -fixed venturi carburetors frequently cause additional inefficiencies that balance with the limitations that they themselves introduce. All the above 10 mentioned and further features attempt to compensate for the inefficiencies ;~
of the venturi over the full range of operating conditions and R.P.M.
Ideally, the carburetor for an automobile should operate at high efficiency `~
over the full range of engine operating R.P.M. and load conditions Conventional air valve type carburetors have been utilized in an attempt to maintain a constant air velocity across a fuel jet oriEice inde-pendent of the throttle valve position and engine speed. Such carburetors generally attempt to vary the size of the fuel jet in accordance with the amount of air entering the carburetor to therefore provide a correct "air-fuel ratio". Engines having such air valve carburetors are generally difficult to start and operate at idling R.P.M.
Each additional compensating feature added to existing carburetors over the years makes them more complicated and more susceptible to plugging and therefore necessitate more frequent repairs and maintenance adjustments.
In addition, the number of moving parts in carburetors has increased substan-tially over the years making such carburetors extremely complicated and ~ -increasingly expensive to manufacture.
The principal reason that variable venturi carburetors have not been a success in the past is because proper metering of fuel at low speeds ~ -has been nearly impossible to maintain. With the engine running, a high vacuum is created in the manifold that is so great that prior variable venturi carburetors would deliver too much gasoline to the engine. The result is that the engine floods too easily or becomes very uneconomical to - 1~5~339~ ~ ~

operate at low speed ranges. ' It is therefore a primary object of this invention to provide a variable venturi carburetor that will effectively operate throughout the , entire engine operation range from idle to full throttle R.P.M. while maintaining a correct air-fuel mixture ratio throughout that range. -~n additional object is to provide such a variable venturi -carburetor wherein the fuel enters the mixing chamber at a point where the aire is moving at an extremely high velocity to thereby thoroughly mix air -and fuel prior to its reception by the engine combustion chambers. :
An additional object is to provide such a variable venturi carbur- r etor wherein fuel is delivered from a single fuel supply orifice throughout ,~
the entire operational range of the engine.
Another principal object of this invention is to provide a carburetor of very simple construction wherein very simple adjustments may be made to vary the fuel-air ratio.
A further object is to provide a single throat carburetor that ,:~
will operate as efficiently as any presently known multiple throat carburetors.
A yet further object is to provide a variable venturi type carbur-etor that does not utilize an idle jet, an accelerator pump, a needle valve, ;
or a butterfly type throttle valve.
An additional object is to provide such a carburetor that is adaptable to replace most conventional downdraft type carburetors.
These and still further objects and advantages will become apparent upon reading the following detailed description of a preferred embodiment.
A preferred embodiment of the present invention is illustrated in the accompanying drawings in which:
Fig. 1 is a vertical cross sectional view taken through the center of the present carburetor;
Fig. 2 is -an enlarged diagrammatic view illustrating operation of the present invention;
Fig. 3 is a view similar to Fig. 2 only showing a different operational condition;

., .

o~3~
, ~:
Fig. 4 is an enlarged pictorial view of the exterior of the present ~
carburetor; ~ ;
Fig. S is a detail plan view of a portion of the present invention; i~
Fig. 6 is an operational detail of a throttle mechanism for the present invention; ~-Fig. 7 is a pictorial view of the first ven~uri member and throttle s; ;
mechanism;
Fig. 8 is an enlarged detailed section view illustrating the variable venturi members at a full throttle condition;
Fig. 9 is a section view taken along line 9-9 in Fig. l; and Fig. 10 is a section view taken along line 10-10 in Fig. 1.
A downdraft variable venturi carburetor embodying the present ~i invention is illustrated in the accompanying drawings and is generally designated therein by the reference numeral 10. The carburetor 10 is designed to be mounted to an internal combustion engine by interconnection to the intake manifold of the engine (not shown). Although the carburetor x illustrated is shown in a single barrel type con-figuration, it should be understood that adaptations may be made to successfully mount the present carburetor on existing manifolds built for two, three or four barrel type -conventional carburetors.
Basically, the carburetor is comprised of a hollow carburetor body 11 that includes an air passage leading Erom an open upper end 12 to an open lower end 13. The body 11 is hollow to direct air from the upper open end 12 into an intermediate mixing chamber 14 and out through lower end 13 and into the intake manifold of an internal combustion engine (not .
shown). A conventional choke 17 is provided across the open upper end 12 to restrict passage of air into the carburetor to facilitate a rich fuel-air mixture for starting purposes in cold weather.
One of the primary features of the present invention is embodied in a fuel dispensing orifice 18 that is annular in configuration and extends about the entire circumference of the mixing chamber 14. Orifice 18 is supp~d with a liquia hydrocarbon fuel through a fuel supply means generally . . ' ' ''' , 5~33~

indicated at 19 and other features of primary importance are a first and a second venturi members 20 and 21. Venturi members 20, 21 are located coaxially within the mixing chamber 14 to define a restricted air passage 22. As illustrated, the second venturi member 21 is formed integrally within carburetor body 11 and defines the annular fuel dispensing orifice 18. The first venturi member 20 is axially movable within mixing chamber 14 along an upright axis through operation of a throttle means 24.
Orifice 18 includes an upper or upstream edge 27 and a downstream axially spaced edge 28 (Fig. 2). These edges bound the portion of orifice 18 openly communicating with mixing chamber 14. ~n enlarged annular cavity 30 is also included as an integral part of orifice 18 and is openly connected to fuel supply means 19. Cavity 30 serves as an initial mixing area for air and fuel before it exits between edges 27 and 28 and into the airstream passing thereby.
As S~lOWn by Figs. 5 and 8, the lower surface of cavity 30 and edge 28 is defined by a removable ring 31 that is formed as a portion of the second venturi member 21. Ring 31 is removable to facilitate selective ~ -adjustment of the restricted air passage size for different performance purposes.
Referring now in greater detail to fuel supply means 19, particular reference will be had to Figs. 1, 9 and 10. Fuel is supplied to the aM ular cavity 30 from a float chamber 44 through means o~ angularly spaced fuel supply tubes 33. Tubes 33 are openly connected to radial ducts 32 that open at ends into chamber 30 at angularly spaced locations.
The individual tubes 33 as shown in Fig. 1 include upper ends 34 that protrude slightly above ducts 32. Lower ends 35 of these tubes are open and extend vertically and axially into float chamber 44. Each tube 33 is centered within a fuel well bore 38 (Figs. 1 and 9) that also projects into float chamber 44.
Means is provided to bleed air into -fuel passing through tubes 33.
This means is in the form of a group of vertically spaced holes 36. The holes 36 are located along tubes 33 so a portion thereof opens into an air space . . . ~ .~
.. ., . . . , . : . ,, . .~ :

:

39~
~ .:
above the fuel level within fuel well bores 38, with the remaining holes being located below the fuel level. As fuel is drawn through tubes 33, air is ~lso drawn through the holes 36 located above the fuel level. The air and , fuel partially~ mix together before the fuel is delivered to the fuel dispensing chamber 30. ;
The fuel well bores 3~ are provided at angularly spaced intervals in a skirt 37 extending into float chamber 44. Bores 3~ extend from upper i;
ends adjacent the upper side of float chamber 44 to lower open ends spaced below the fuel level. Near the upper closed ends of each bore 38 is a spaced pair of holes 39. These are pressure regulating holes that bleed air from the float chamber to the wells to compensate for varying atmospheric pressure and for partially regulating the mixture of fuel and air passing to the orifice 18. The function of bores 38 and holes 39 will be discussed in greater detail in the ollowing d~scussion of the operation of the present invention.
~uel is supp~d to float chamber 44 and maintained at a prescribed level therein by a float mechanism 47. ' ;
The air space 45 in float chamber 44 communicates with atmospheric air through means of a ventilation tube ~9. Tube 49 openly communicates , 20 with atmospheric air from an open end protruding into the hollow interior of '; ' the carburetor above the restricted air passage. Air pressure within chamber 44 is thereby held equal to the outside atmospheric air pressure.
In addition to air being bled into the fuel through holes 39, a number of air bleed holes 51 are also provided in second venturi member 21 ;~
to bleed air into the orifice cavity 30. Bleed holes 51 are utilized both to assist in premixing the fuel and air and as a means for regulating the air pressure in the orifice chamber 30. An ad~usting means 52 is provided in the form of threaded screws 53 to selectively control the amount of air bled through the bleed holes 51. Screws 53 are carried within complementary threaded apertures within the carburetor body 11 intersecting the holes 51. `-As shown in Fig. ~, several such screws are provided with one for each air bleed hole 51. It is presently contemplated to utilize a plurality of such ~-~.~S~39~

holes at equiangularly spaced positions about the central carburetor axis.
These holes 51 play an important role in the prsper functioning of my invention and will be discussed in greater, detail at a later point in the description of the operation.
Reference will now be made in particular de~ail to the first venturi member 20. ~his member is shown in detail by Figs. 1, 7, and 8. i~
Venturi member 20 is bell-shaped in configuration starting -from a central `-upper reduced end 55. A concave portion 56 extends downward and radially ~.
outwardly from reduced end 55. Concave portion 56 ends in a downstream edge 57 from where an axial rim 58 extends. Axial rim 58 projects axially downward from edge 57 to a lower rim edge 59. As shown in ~ig. 1~ the structure including concave portion 56 and axial rim 58 may be provided independent of the remainder of venturi member 20. However, it is not critical that this be so and is entirely feasable that these portions be formed integrally along with the remaining portions of venturi member 20.
An outwardly beveled portion 60 extends dcwn and outwardly from the lower rim edge 59 to a point of maximum diameter -from the axis. From this point, an inwardly beveled portion 61 extends on downstream to terminate at a bottom venturi edge.
Another important feature is the provision of a plurality of bleed holes 63 formed through concave portion 56. These holes openly .
cGmmunicate with an open chamber 64 within venturi member 20. ~loles 63 are located radially inward of axial rim 58 to receive air from a point upstream of restricted air passage 22 and to direct it into the chamber 64 Included within axial rim 58 are a plurality of holes 65 and a number of slots 66 that communicate openly with cha~ber 64. It may be noted in Figs. 2, 3, 7 and 8, that the holes 65 are located above slots 66. Again, this constitutes an important feature concerning the operation of this invention and will be discussed in greater detail below.
Pirst venturi member 20 is movably held along the central car-buretor axis by throttle means 24. Included with the throttle means 24 is a central support body 67 that slidably receives a downwardly open cup shaped ~ 3~ :

portion 67_ of venturi member 20 that includes beveled portions 60 and 61.
The hollow portion 67a is vented through upright holes 71 to rel~ève air pressure while venturi member 20 moves on body 67. A central upright shaft 68 extends downwardly from member 20 through an appropriate aperture -formed through body 67. This shaft 68 slidably communicates with a cam 69 provided .;
by throttle means 24. This cam is shown in detail by Fig. 6.
A linkage rod 70 is operatively connected to cam 69 and extends through the carburetor body 11 to communicate with a known form of throttle linkage ordinarily provided ~n internal combustion engines. The sha-ft 68 is rotatably journalled by the carburetor body to enable rotational movement of the cam 69 between positions shown by dashed and solid lines in Fig. 6. `
This movement of cam 69 results in corresponding up and downward movement of the first venturi member 20. Relative positions of venturi m~mber 20 with respect to second venturi member 21 may be seen by comparing Pigs. 2, 3, and 8. Fig. 2 represents an idle position while Fig. 8 represents a full :
throttle setting. -As previously recited, the second venturi member 21 is formed integrally with the carburetor body 11. Venturi 21 is shown in substantial detail by Figs. 1 and 8. A concave portion 72 of venturi 21 leads tangentially downwardly from an edge above the restricted air passage. This portion 72 leads into a convex portion 73 that connects tangentially thereto.
Convex portion 73 leads on downward to the upper orifice edge 27, Extending from the lower edge 28 then is a second axial rim 74. , Rim 74 is complementary to and slightly larger in diameter than the ~.
corresponding rim 58 of first venturi member 20. Rim 74 leads downwardly to an outwardly beveled portion 75 that is complementary to the first beveled portion 60 of the first member 20. The beveled portions 60 and 75, along with rims 74 and 58 define the restricted air passage immediately adjacent to the orifice 18 during idle and high idle RPM as shown by Figs. 2 and 3.
A flared portion 76 extends out and downwardly from beveled portion 75 -to tangentially join with the remaining portion of the carburetor interior.

~LC~5~ 3~
Figs. 5 and 8 show particular details of second venturi member 21 between lower orifice edge 28 and the lower edge o-f outwardly beveled ~ -portion 75. There is provided in this part of the second venturi member 21, a plurality of axial grooves 78 and slots 82. The axial grooves 78 are equiangularly spaced about the inside periphery of venturi member 21 and extend from top edges 79, openly communicating with orifice 18, to closed bottom edges 80. The edges 80 terminate at the juncture of beveled portion 75 and flared portion 76.
Also included with the present carburetor 10 is a water jacket 87.
Jacket 87 extends about the periphery of the hollow carburetor interior adjacent fuel dispensing orifice l8. The jacket includes an infeed 88 and an outlet 89 adapted to receive a coolant from an engine's cooling system.
Through this manner, the area in the vicinity of orifice 18 is heated by conduction. Water jacket 87 prevents icing within the carburetor at the points of highest air velocity adjacent orifice 18. It has been my experience that without provision of such a jacket 87, the extremely high velocity air will soon cause icing within the carburetor and result in poor operational efficiency.
Operation of the present invention may now be understood rom the foregoing technical disclosure. First, prior to operation, the carburetor is installed on an existing engine with the lower end 13 bolted over the exist- ;
ing carburetor mounting surface of the intake manifold. A fuel line is then connected to float chamber and choke 17 and throttle means 2~ are connected to the conventional throttle and choke linkage provided by the engine.
To start the engine, the throttle linkage is operated to move ~ i first venturi member 20 to a position adjacent the fuel dispensing orifice lS
as is shown in Figs. 1 and 2. In this condition, -the concave and convex portions of venturi members 20 and 21 funnel air from an upstream location downwardly toward and across the orifice 18. The engine then first pulls air and fuel through grooves 78 from the fuel supply means 19. This relatively rich mixture is ordinarily all that is required to start the engine running under its own power.

,`' ' , ' 93~
;~, . .
Once the engine is running, the vacuum within the manifold becomes so great that ordinary variable venturi carburetors would respond by delivering an excessively rich mixture of fuel and air to the engine. The present carburetor, however, avoids this problem by supplying air to the engine through several different routes.
In the low throttle condition, some idle air will enter the cavity 30 between the edges 27 and 28. This is accomplished as air is received `
through bleed holes 63 and directed radially outward through holes 65 that are presently aligned with the aperture 18. 'rhis provision serves to ~ -substantially reduce the vacuum within the cavity 30 an~ thereby controls the delivery of fuel from means 19.
In addition to the above feature, air is bled into oriEices 18 through air bleed holes 51 provided within the second venturi member 21. The low pressure within cavity 30 is such that air is drawn downwardly ~hrough holes 51 and into orifice l8. The provision of adjusting means 52 facilitates adjustment of the idle air supply to cavity 30 and thereby will facilitate selective control of the air fuel mixture at the idle settings. ;;~
Features including the air bleed holes 51 and holes 63 and 65 will successfully meter the intake of fuel and air mixture while enabling fuel delivery through the single annular fuel dispensing orifice 18. Therefore, the fuel and air are brought together at a point where the air reaches its highest velocity. This serves to completely atomize and simultaneously mix the fuel and air together to develop a completely mixed charge for delivery to the engine cylinders.
In the idling position, fuel is drawn from orifice 18 through the upright axial grooves 78, partly through engine vacuum and partly by the tidal rush of air passing between surfaces 60 and 75.
To increase the speed of the engine from a slow idle to a fast idle, the throttle means is operated to slightly lower first venturi member 20 axially within mixing chamber 14. The downward force of air against , first venturi member 20 facilitates its movement downward without the assistance of springs or any o~her attachment mechanism to the throttle -10- ;~ , j .

~C~3391 linkage.
As venturi member 20 moves elevationally downward, the edge 57 of concave surface 56 moves downward toward alignment with the lower edge 28 of ~`
orifice 18. Excessive fuel is prevented from being pulled from the orifice 18 (as air passage 22 is enlarged) by the configuration of convex surface 56 `
and the axial alignment of orifice edge 27 and rim 58. In this c~ndition, air is directed radially by concave surface 56 into the opening 18 to prevent the excessive withdrawal of fuel through supply means 19.
To increase ~he engine speed above a fast idle, first venturi 20 is again lowered so edge 57 is dropped below lower edge 28 of orifice 18. In this position, excessive fuel is again prévented from entering the main air stream as additional air is pulled directly into the top of slots 78.
An additional increase in engine's rate of speed brings the first venturi member 20 to the position shown by Fig. 3. In this position, the axial grooves 78 are not as effective because they are drawing considerably less air from orifice 18 and more air through slots 66 and holes 65. The fuel mixture is prevented from becoming too lean as the tidal rush of air passing orifice 18 creates a law pressure across the orifice opening. This low pressure serves to pull a mixture of fuel and air from supply means 19 into the air stream which now passes through grooves 78 as well as slots 82 in second venturi member 21 and holes 65, and the slots 66 provided in first venturi member 20. `
As first venturi member 20 continues to move downwardly, air passage 22 is enlarged and air pressure is continually lowered at orifice 18 to pull additional fuel as air mcves by at increasing velocity.
During high speed or full throttle operation, rim 58 is displaced vertically ~axially~ away from corresponding rim 74. In this conditian, grooves 78 and slots 82 become ineffective for the purpose of metering fuel.
A full throttle position of first and second venturi members 20, 21 is illustrated in Fig. 8. In this position, the concave curvature 56 directs air directly past orifice 18. In addition to the low pressure created at orifice 18, a suction force is produced as air passes at high velocity over .,:

. " .

.. ~ . . . ,., . . . ~ .. . : , ., . ;. , , . . . . . . .:
.... ,, . ,..... ~; ., .,,, ". . . . ... ~/ .

~ 3~059391 ~

,, bleed ducts 51 along concave portion 72 of second venturi member 21. High velocity air passing these orifices creates low pressure across their openings, `
which is transferred to cavity 30. This additional low pressure serves to i:
..~,. .... . .
draw more fuel through the supply means 19 in order to provide an appropriate rich mixture at the full throttle setting.
Low pressure within orifice 18 results in subsequent pressure -drops within fuel well cavities 38. It is a function of holes 36 to respond to pressure differential within orifice 18 by drawing corresponding amounts s -of fuel and air from float chamber 449 from within fuel well bores 38. -~
Bores 38 serve as individual fuel wells for each tube 33. As air pressure in fuel well bores 38 drops, the fuel level will rise to successively ~^
c wer more bleeder holes 36. This allows less air to be bled into the fuel ; -passing through the tubes 33. They therefore deliver a rlcher fuel mixture to orifice 18.
The level to which the fuel will rise within bores 38 is determined by the remainder of uncovered holes 36 within tubes 33. The fuel will remain at any level as long as appropriate vacuum pressure is maintained. -This is because the volume of air being bled into the remainder o holes 36 . ., is replaced by air being drawn into bores 38 through holes 39.
The fuel level in each bore 38 will fluctuate as the pressure within orifice 18 fluctuates. In addikion~ the fuel level within the bores 38 willfluctuate as atmospheric pressure changes. This eature automatically meters the right amount of fuel to correspond with changes in altitude. `
It is understood that various changes and modi-fications may be made in the above disclosure without departing from the intended scope of ; this invention. Therefore, only the following claims are to be taken as definitions or limitations upon the scope of the present invention.

~;''" , .' , .

~'; "
.. , .. , . . . . . ~ . . . ,: . . .. .

Claims (22)

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

1. A downdraft variable venturi carburetor for an internal combustion engine, comprising:
a hollow carburetor body having an upwardly open air intake end and a downwardly open end adapted to be mounted to an intake manifold of an internal combustion engine and a fuel-air mixing chamber intermediate the ends for receiving and guiding air in a downstream flow from the intake and through the downwardly open end and into the manifold;
an annular fuel dispensing orifice encircling the fuel mixing chamber within the body;
means for supplying fuel to the orifice at a plurality of angularly spaced locations in response to operation of the engine;
a first movable venturi member located centrally within the body along an upright axis for axial movement therein within the mixing chamber;
a second venturi member defining the orifice and located coax-ially with and radially adjacent to the first venturi member;
wherein the first and second venturi members together form an annular restricted air passage downwardly adjacent to the fuel dispensing orifice; and throttle means for axially moving the first venturi member relative to the second venturi member to enlarge and reduce the restricted air passage and thereby increase and decrease the flow of air and fuel through the mixing chamber.

2. The carburetor as defined by claim 1 wherein the first venturi member is substantially bell-shaped and includes:
a reduced end facing the intake end;
a concave portion curving down and outwardly from the reduced end to a downstream edge;

a first axial rim extending downstream from the downstream edge of the concave portion;
a first outwardly beveled portion extending downstream from the first axial rim; and an inwardly beveled portion extending downstream from the first outwardly beveled portion.

3. The carburetor as defined by claim 2 wherein the second venturi member is integral with the carburetor body and includes:
a convex portion defining an upstream edge of the orifice and leading toward the open intake end;
a second axial rim portion defining a downstream edge of the orifice complementary to the first axial rim portion of the first venturi member and spaced radially outward therefrom; and a second outwardly beveled portion extending downstream from the second axial rim complementary to the first outwardly beveled portion.

4. The carburetor as defined by claim 3 further including open air passages within the first venturi member extending from upstream openings within the concave portion to downstream openings within the first axial rim.

5. The carburetor as defined by claim 3 further comprising:
a second concave portion joined tangentially to the concave portion and extending upstream therefrom toward the open upper end; and wherein the second venturi member further includes an axial air bleed passage leading downstream from an end opening on the second concave portion to a remaining end opening into the fuel dispensing orifice.

6. The carburetor as defined by claim 5 further including adjusting means for selectively limiting the amount of air flow through the axial air passage.

7. The carburetor as defined by claim 4 further comprising:
a second concave portion joined tangentially to the concave portion and extending upstream therefrom toward the open upper end; and wherein the second venturi member further includes an axial air bleed passage leading downstream from an end opening on the second convex portion to a remaining end opening into the fuel dispensing orifice.

8. The carburetor as defined by claim 7 further including adjusting means for selectively limiting the amount of air flow through the axial air passage.

9. The carburetor as defined by claim 3 further comprising:
an open axial groove formed in the second venturi member facing the first venturi member and leading downstream from an upper end openly communicating with the fuel dispensing orifice to terminate at a closed end downstream of the second outwardly beveled portion.

10. The carburetor as defined by claim 3 further comprising:
an open slot formed in the second rim and outwardly beveled portion of the second venturi member extending axially from an end downstream of the downstream orifice edge to terminate at a location along the second outwardly beveled portion.

11. The carburetor as defined by claim 9 further comprising:
an open slot formed in the second rim and outwardly beveled portion of the second venturi member extending axially from an end downstream of the downstream orifice edge to terminate at a location along the second outwardly beveled portion.

12. The carburetor as defined by claim 4 further comprising:
an open axial groove formed in the second venturi member facing the first venturi member and leading downstream from an upper end openly communicating with the fuel dispensing orifice to terminate at a closed end downstream of the second outwardly beveled portion.

13. The carburetor as defined by claim 12 further comprising:
an open slot formed in the second rim and outwardly beveled portion of the second venturi member extending axially from an end downstream of the downstream orifice edge to terminate at a location along the second outwardly beveled portion.

14. The carburetor as defined by claim 1 further comprising a water jacket formed about the carburetor body adjacent the fuel dispensing orifice, adapted to be connected to an internal combustion engine water cooling system.

15. The carburetor as defined by claim 1 wherein the throttle means is comprised of a cam slidably engaging the first venturi member and operated by a rod pivotably journalled within the carburetor body.

16. The carburetor as defined by claim 1 wherein the fuel supply means is comprised of:
an annular float chamber disposed downstream of the fuel dispensing orifice;
float regulated means adapted to be connected to a fuel supply source for maintaining a preselected level of fuel within the float chamber with an air space above;
a plurality of upright axial fuel delivery tubes extending from upper ends opening into the fuel dispensing orifice to lower open ends below the preselected fuel level.

17. The carburetor as defined by claim 16 wherein the fuel supply means includes means for bleeding air into the fuel supply tubes to premix air with fuel before it is delivered to the fuel dispensing orifice.

18. The carburetor as defined by claim 16 wherein the air bleed means is comprised of a plurality of apertures in each tube spaced apart along its length with some of the apertures located within the air space and the remainder located below the selected fuel level.

19. The carburetor as defined by claim 18 wherein the carburetor body includes an annular skirt including a plurality of axial downwardly open fuel well bores extending into the float chamber from closed ends located above the selected fuel level to open ends located below the selected fuel level;
apertures extending through the skirt from the fuel well bores adjacent the closed ends thereof to openly communicate with the air space within the float chamber; and wherein the fuel delivery tubes are centrally located within the fuel well bores.

20. The carburetor as defined by claim 18 further comprising vent means for exposing the float chamber to outside atmospheric pressure.

21. The carburetor as defined by claim 20 wherein the vent means is comprised of a ventilator duct openly connecting the carburetor bore with the air space in the float chamber.

22. The carburetor as defined by claim 21 wherein the ventilator duct extends from one end opening into the air space to a remaining end opening into the bore at a location upstream of the venturis.