Variable venturi type carburetor

A variable venturi type carburetor comprising a carburetor body provided with a suction passage for flow of air therethrough, a slide valve supported by the body for slidable movement across the suction passage to serve as a variable venturi and a butterfly throttle valve pivotably supported by the carburetor body downstream of the slide valve. An interlocking mechanism connects the slide valve and butterfly throttle valve together for operating in correspondence with one another and one of the valves is operated by application of an external force thereto. A low-speed fuel nozzle opens into the suction passage in the vicinity of the butterfly throttle valve, an intermediate- and high-speed main fuel nozzle opens into the suction passage opposite the slide valve, and a low- and intermediate-speed primary fuel nozzle opens into the suction passage between the slide valve and the butterfly throttle valve. The lower edge of the slide valve is formed with an inverted cutaway to provide a widened passage facing in the downstream direction of the suction passage.

FIELD OF THE INVENTION 
This invention relates to a carburetor of variable venturi type and 
associated method of operation. 
DESCRIPTION OF PRIOR ART 
In conventional carburetors of variable venturi type a slide throttle valve 
capable of being moved slidingly across a suction passage is operated by a 
throttle wire. In such carburetor, the throttle slide valve is subjected 
to a force acting downstream, in the suction direction of air flow, due to 
the vacuum produced in the engine. Consequently, a relatively large 
frictional force is developed between the side surface of the slide 
throttle valve which faces in the downstream direction and the opposed 
surface of the carburetor body. Therefore, a relatively large tractive 
force is necessary to operate the throttle wire. 
A variable venturi carburetor of so-called constant-vacuum type has also 
been developed in an effort to eliminate these deficiencies. In this 
carburetor the vacuum is controlled by means of a butterfly throttle valve 
provided in the suction passage and the slide throttle valve is opened and 
closed in accordance with the resulting vacuum. However, if the open 
degree of the butterfly throttle valve in this carburetor is increased 
suddenly, the vacuum does not increase accordingly. In consequence, the 
action of the slide throttle valve does not follow the sudden acceleration 
operation. Thus, this variable venturi type carburetor has a low 
acceleration response 
The present inventor has already proposed a variable venturi type 
carburetor which is intended to eliminate these deficiencies. In this 
carburetor, the butterfly throttle valve and the slide valve are 
operatively connected for operation in correspondence with one another, 
and a low-speed fuel discharge port and a main fuel nozzle are 
respectively provided in the vicinity of the butterfly throttle valve and 
just under the slide valve. According to this arrangement, the 
acceleration response of the slide valve can be improved. In addition, the 
discharge rate of fuel from the low-speed fuel discharge port can be 
controlled properly in the low-load operational region, and the discharge 
rate of fuel from the main fuel nozzle can be controlled properly in the 
high-load operational region. 
In the carburetor of the above-described construction, the slide valve and 
the butterfly throttle valve are moved by operating a throttle wire by 
application of external force thereto. Accordingly, the vacuum in the 
suction passage does not increase in accordance with sudden opening 
operations of these two valves under certain operational conditions. In 
such case, the discharge rate of fuel from the main fuel nozzle becomes 
insufficiently low in the region of an intermediate degree of opening of 
the slide valve. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a construction which 
avoids the deficiencies of the known carburetor. 
It is a particular object of the present invention to provide a variable 
venturi type carburetor capable of producing an excellent air-fuel ratio 
in all operational regions, i.e. from the low-load operational region to 
the high-load operational region. 
In order to satisfy the above and further objects of the invention, a 
carburetor is provided, which comprises a carburetor body having a suction 
passage therein, a slide valve supported by said body for slidable 
movement across said suction passage to function as a variable venturi, a 
butterfly throttle valve pivotably supported by the carburetor body 
downstream of the slide valve, interlocking means connecting the slide 
valve and the butterfly throttle valve together for operation in 
correspondence with one another, operating means connected to one of said 
valves for operating the same by application of external force thereto, a 
low-speed fuel nozzle opening into the suction passage in the vicinity of 
the butterfly throttle valve, an intermediate and high-speed main fuel 
nozzle opening into the suction passage just under the slide valve, and a 
low and intermediate-speed primary fuel nozzle opening into the suction 
passage between the slide valve and the butterfly throttle valve. 
According to this arrangement, the discharge rates of fuel from the 
low-speed fuel nozzle, the main fuel nozzle and the primary fuel nozzle 
can be controlled properly in a low-load operational region, 
intermediate-and high-load operational regions and a transitional region, 
in which low-load operation of the engine is shifted to intermediate and 
high-load operations respectively. Therefore, an excellent air-fuel ratio 
can be obtained in all operational regions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
An embodiment of the present invention will now be described with reference 
to the drawing wherein a float chamber body 4 forming a float chamber 3 is 
secured via a seal member 5 to a lower portion of a carburetor body 2 in 
which a suction passage 1 is formed. In suction passage 1 is a slide valve 
6 adapted to be moved slidingly across the suction passage 1, and a 
butterfly throttle valve 7 pivotably supported by the carburetor body 2 on 
the downstream side of the slide valve 6 with respect to the direction of 
air flow 8, i.e., the suction direction. The slide valve 6 and the 
butterfly throttle valve 7 are operated correlatively from their 
fully-closed positions to their fully-opened positions. 
The carburetor body 2 is provided with an intermediate and high speed main 
fuel nozzle 10 which opens at the inner surface of the suction passage 1. 
An air bleeder pipe 11 is connected to a lower portion of the main fuel 
nozzle 10 integrally and concentrically. A main fuel jet 12 extending 
under the fuel level in the float chamber 3 is joined to a lower portion 
of the air bleeder pipe 11. Thus, a main fuel passage Mw extending from 
the main fuel jet 12 to the main fuel nozzle 10 via the air bleeder pipe 
11 is formed. The main fuel passage Mw opens into the suction passage 1 
just under the slide valve 6. An annular chamber 17 formed around the air 
bleeder pipe 11 is in communication with an upstream end of the suction 
passage 1 via an air bleeder passage (not shown). 
The carburetor body 2 is further provided with a low speed fuel passage Sw 
which opens into the suction passage 1 in the vicinity of the butterfly 
throttle valve 7. A pilot outlet 18, which opens into the suction passage 
1 on the slightly downstream side of the butterfly throttle valve 7, and a 
low speed fuel nozzle 19, which opens into the suction passage 1 on the 
slightly upstream side of the butterfly throttle valve 7 in its 
fully-closed position are also provided in the carburetor body 2. The 
pilot outlet 18 and the fuel nozzle 19 are in communication with a fuel 
passage 20. A low speed fuel jet 21, which extends under the fuel level in 
the float chamber 3, is connected to the fuel passage 20 via an air 
bleeder pipe 22. In order to regulate the degree of opening of the pilot 
outlet 18, a pilot screw 23 is engaged with the carburetor body 2 so that 
the pilot screw 23 can be turned to advance upwardly and downwardly. 
The carburetor body 2 is further provided, at its lower portion, with a low 
and intermediate speed primary fuel nozzle 28 which opens into the suction 
passage 1 between the slide valve 6 and the butterfly throttle valve 7. An 
air bleeder pipe 29 is connected to a lower portion of the primary fuel 
nozzle 28 integrally and concentrically. A primary fuel jet 30 which 
extends under the fuel level in the float chamber 3 is joined to a lower 
portion of the air bleeder pipe 29. An annular chamber 31 formed around 
the air bleeder pipe 29 is in communication with an upstream end of the 
suction passage 1 via an air bleeder passage (not shown). 
A float 13 is housed in the float chamber 3. A float valve 14 is engaged 
with a pivotably supported portion of the float 13 so as to open and close 
a valve port 16 in accordance with vertical movement of the float 13. The 
valve port 16 is in communication with a fuel supply passage 15 formed in 
the carburetor 2. 
A guide cylinder 24 extends upwardly at an upper portion of the carburetor 
body 2 at a location opposite the main fuel nozzle 10. The guide cylinder 
is integral with the carburetor body 2. A housing 26 forming an air 
chamber 25 is integrally joined to an upper portion of the guide cylinder 
24. The air chamber 25 is in communication with an upstream end of the 
suction passage 1 via a passage 27. 
The slide valve 6 is formed in the shape of an open top, closed bottom 
cylinder, and the valve 6 is fitted slidably in the guide cylinder 24. A 
needle valve 34 is secured to the bottom of the slide valve 6 and is 
inserted into the main fuel nozzle 10. An upwardly extending recess 32 is 
provided in the lower end surface of the slide valve 6 and an inverted 
cutaway 33 is formed in the side surface at the bottom of slide valve 6 on 
the downstream side with respect to the suction direction 8. The recess 32 
thus provided causes turbulence to occur in the air flow therein, so that 
the vacuum applied to the main fuel nozzle 10 can be made uniform. The 
cutaway 33 enables the vacuum in the space between the bottom portion of 
the slide valve 6 and the inner surface of the wall of the suction passage 
1, i.e., a venturi portion, to increase. Consequently, the discharge rate 
of fuel from the main fuel nozzle 10 increases, and the regulation of the 
air-fuel ratio can be easily effected. 
A shaft 43 which extends parallel to a valve shaft 39 of the butterfly 
throttle valve 7 is pivotably supported in the housing 26, and a driving 
arm 44 is connected fixedly at one end thereof to the pivotable shaft 43 
in the air chamber 25. A bracket 45 is connected fixedly to the slide 
valve 6. The bracket 45 is also connected at the other end thereof to the 
other end of the driving arm 44 by a connecting rod 46. Accordingly, 
reciprocating pivotal movements of the pivotable shaft 43 are converted 
into linear reciprocating movements of the slide valve 6 along the guide 
cylinder 24, i.e., the opening and closing movements of the slide valve 6, 
via the driving arm 44, connecting rod 46 and bracket 45. 
Referring to FIGS. 2-3, the valve shaft 39 of the butterfly throttle valve 
7 and the pivotable shaft 43 are connected together via an interlocking 
mechanism 9 so as to correlate the opening and closing actions of the 
slide valve 6 with those of the butterfly valve 7. The interlocking 
mechanism 9 is arranged in a housing chamber 60 provided at a side portion 
of the carburetor body 2. The housing chamber 60 is defined by a wall of a 
housing recess 61 provided at a side portion of the carburetor body 2, and 
a cover member 62 fastened to the carburetor body 2 so as to close the 
housing recess 61. 
The interlocking mechanism 9 consists of a throttle lever 47 press-fitted 
firmly around an end portion of the valve shaft 39, a pivotable arm 48 
mounted on an end portion of the pivotable shaft 43, and a connecting arm 
49 fixed at one end to the pivotable arm 48 and joined at the other end 
thereof to the portion of the throttle lever 47 which is remote from the 
axis thereof. A regulator mechanism 50 is interposed between the pivotable 
arm 48 and the pivotable shaft 43. A throttle wire 41 is connected to the 
throttle lever 47. When the throttle wire 41 is drawn in the direction of 
arrow 42, the butterfly throttle valve 7 is turned in the opening 
direction. The butterfly throttle valve 7 is urged in the closing 
direction by a coil spring 40 so that when the tractive force of the 
throttle wire 41 is decreased, the butterfly throttle valve 7 is turned in 
the closing direction. The opening and closing actions of the butterfly 
throttle valve 7 are transmitted to the pivotable shaft 43 via the 
interlocking mechanism 9 and the regulator mechanism 50 so that the slide 
valve 6 is opened or closed in accordance with the pivotal movement of the 
shaft 43. 
The regulator mechanism 50 consists of a lever 52 which is mounted on an 
end portion of the pivotable shaft 43 so that the lever 52 is angularly 
fixed on the shaft 43 and extends in the same direction as the pivotable 
arm 48, a projection 53 provided on the lever 52, and a coil spring 55 
urging the lever 52 to turn in the direction in which the projection 53 
comes into contact with the pivotable arm 48. The coil spring 55 is fitted 
around the pivotable shaft 43 and is engaged at one end with an integral 
pin 56 in the housing 26, and at the other end with the lever 52. The 
pivotable arm 48 is fitted at its base portion around the pivotable shaft 
43 so that the arm 48 can be turned relative to the shaft 43. A setting 
nut 57 is fixedly secured at an end of the pivotable shaft 43 so as to 
prevent the pivotable arm 48 from coming off from the shaft 43. The 
pivotable arm 48 is provided with a contact arm 58 which is capable of 
regulating the circumferential distance between the arm 58 and the portion 
of the pivotable arm 48 to which the connecting arm 49 is joined. The 
contact arm 58 is provided with a projection 59 engageable with the 
projection 53. 
In the interlocking mechanism 9 and the regulator mechanism 50, which are 
formed as described above, the operation of the throttle lever 47 for 
opening the butterfly throttle valve 7, i.e., clockwise pivotal movement 
in FIG. 2 of the lever 47, is transmitted to the pivotable arm 48 to cause 
the arm 48 to turn clockwise. Since the projection 53 in the regulator 
mechanism 50 is engaged resiliently with the projection 59 of the 
pivotable arm 48, the lever 52 and the shaft 43 are turned clockwise. The 
pivotal movement of the shaft 43 is transmitted to the slide valve 6 via 
the driving arm 44, connecting rod 46 and bracket 45, so that the slide 
valve 6 is displaced upwardly along the guide cylinder 24, i.e., moved in 
the opening direction. 
Conversely, when the butterfly throttle valve 7 is turned counterclockwise 
in FIG. 2, the pivotable arm 48 is also turned counterclockwise. In 
accordance with the counterclockwise movement of the pivotable arm 48, the 
lever 52, i.e., the pivotable shaft 43 turns counterclockwise by the 
resilient force of the coil spring 55 as the projection 53 follows the 
projection 59 in a contacting state. Consequently, the slide valve 6 is 
forced downwardly via the driving arm 44, connecting rod 46 and bracket 
45, i.e., moved in the closing direction. At this time, the pivotable arm 
48 can be turned counterclockwise by the regulator mechanism 50. 
Therefore, the butterfly throttle valve 7 can be closed irrespective of 
the movement of the slide valve 6. 
The regulator mechanism 50 is capable of finely regulating the degree of 
opening of the slide valve 6 with respect to that of the butterfly 
throttle valve 7 by regulating the distance between the portion of the 
pivotable arm 48 to which the connecting arm 49 is joined and the contact 
arm 58. Since the projection 53 resiliently engages the projection 59, any 
vibration of the throttle lever 47, pivotable arm 48 and connecting arm 
49, due to mounting errors is damped so that the interlocking mechanism is 
operated smoothly. 
The throttle lever 47 is provided with a limit projection 63 extending 
laterally therefrom. A stop screw 64 is engaged in a threaded bore 71 in a 
boss 72 formed integrally with the cover member 62, so as to contact the 
limit projection 63. A loosening-preventing portion 75, opposed to an end 
surface of the pivotable shaft 43, projects from the cover member 62. The 
loosening-preventing portion 75 is adapted to engage setting nut 57 and 
prevent the same from being loosened. A cap 73 is engaged with an upper 
portion of the wall of the housing recess 61, and an end portion of an 
outer wire 74 is fixedly secured in the cap 73. The throttle wire 41 which 
can be moved through the outer wire 74 is connected to the throttle lever 
47 within the housing chamber 60. 
The operation of this embodiment will now be described. 
In accordance with the opening and closing actions of butterfly valve 7 by 
drawing the throttle valve wire 41, the slide valve 6 is opened and closed 
via the interlocking mechanism 9. During this time, the suction vacuum 
does not directly cause the slide valve 6 to be drawn in a downstream 
direction since the butterfly throttle valve 7 is provided on the 
downstream side of the slide valve 6. Accordingly, the frictional 
resistance between the outer surface of the slide valve 6 and the inner 
surface of the guide cyliner 24 is comparatively low, so that the throttle 
wire 41 can be operated by a comparatively small tractive force. Moreover, 
when the opening degree of the butterfly throttle valve 7 is increased 
suddenly for sudden acceleration of the engine, the slide valve 6 is 
opened without delay and excellent acceleration can be obtained. 
In the case where the opening degree of the butterfly throttle valve 7 is 
set to a low level to carry out low-load operation of the engine, the 
discharge rate of fuel from the low-speed fuel nozzle 19 can be controlled 
in accordance with the opening degree of the valve 7 since the nozzle 19 
is provided in the vicinity of the valve 7 and the discharge rate can be 
controlled with high accuracy. 
When the opening degree of the slide valve 6 is set to an intermediate or 
high level so as to operate the engine with an intermediate or high load, 
the slide valve 6 carries out its venturi effect to control the vacuum 
above the main fuel nozzle 10 in accordance with the load. The discharge 
rate of fuel from the main fuel nozzle 10 is thus regulated to enable the 
production of a fuel mixture suitable for intermediate and high-load 
operations of the engine. 
When the slide valve 6 is opened suddenly to shift a low-load operation of 
the engine to an intermediate-load operation thereof, the vacuum in the 
suction passage 1 does not increase accordingly in some cases. In such 
cases, there is the possibility that the discharge rate of fuel from the 
main fuel nozzle 10 becomes insufficiently low. If this occurs, the vacuum 
in the portion of the suction passage 1 which is between the butterfly 
throttle valve 7 and the slide valve 6 becomes greater than that below the 
slide valve 6. Since the low and intermediate-speed primary fuel nozzle 28 
opens into the suction passage 1 between the butterfly throttle valve 7 
and the slide valve 6, the fuel is discharged from the nozzle 28 so as to 
compensate for the shortage of fuel discharged from the main fuel nozzle 
10. 
Thus, an excellent air-fuel ratio can be obtained in all operational 
regions of the engine, i.e. from the low-load operational region to the 
high-load operational region. 
According to the present invention as described above, a novel carburetor 
is provided, which comprises carburetor body 1 provided with suction 
passage 2, slide valve 6 slidingly movable across the suction passage and 
functioning as a variable venturi, butterfly throttle valve 7 pivotably 
supported on the carburetor body downstream of the slide valve, 
interlocking mechanism 9 connecting the slide valve and the butterfly 
throttle valve for corresponding movement together, operating member 41 
connected to one of the valves (valve 7 in the embodiment) to operate the 
valve by application of external force thereto, low-speed fuel nozzle 19 
which opens into the suction passage in the vicinity of the butterfly 
throttle valve, intermediate and high-speed main fuel nozzle 10 which 
opens into the suction passage just under the slide valve 6, and low- and 
intermediate-speed primary fuel nozzle 28 which opens into the suction 
passage between the slide valve and the butterfly throttle valve. 
Therefore, in a low-load operational region, the flow rate of fuel and the 
air-fuel ratio are controlled properly by the butterfly throttle valve, 
and, in the intermediate and high operational regions, the discharge rate 
of fuel from the main fuel nozzle is controlled by the slide valve. When 
the operating member suddenly opens the valve to which it is connected to 
go from a low-load operation of the engine to an intermediate or high-load 
operation thereof, fuel is discharged from the primary fuel nozzle 28 to 
compensate for the shortage of fuel discharged from the main fuel nozzle 
10. Accordingly, the discharge rate of fuel does not become insufficiently 
low and an excellent air-fuel ratio can be obtained in all operational 
regions of the engine, i.e. from the low-load operational region to the 
high-load operational region thereof. 
Although the invention has been described in relation to specific preferred 
embodiments thereof, it will become apparent to those skilled in the art 
that numerous modifications and variations can be made without departing 
from the scope and spirit of the invention as defined in the attached 
claims.