CARBURETOR CIRCUITS AND ITS PURPOSE AND OPERATION
Since all late model cars and light trucks are using downdraft carburetors, our text will apply only to them. However, the principles of carburetion do not change. The only difference between the terms updraft and downdraft, is the direction in which air enters the carburetor throat or air horn. It either enters from the bottom and travels up into the engine, or from the top down into it. In as much as updraft carburetors are in the minority as far as equipment on present day cars and trucks is concerned, it is of little value to discuss them in this book. All downdraft carburetors regardless of size and shape operate in the same general manner. This being the case, we can break their construction into five separate circuits for easier discussion and understanding. The circuits are as follows: FLOATING CIRCUIT CHOKE CIRCUIT IDLE SPEED CIRCUIT LOW SPEED CIRCUIT MAIN METERING CIRCUIT FULL SPEED CIRCUIT AC CE LE R AT E CI RC UI T
Since we have listed the circuits in sequence, let us discuss them in this manner. In the part of the carburetor known as the body is located the float bowl or chamber. This chamber is used for the storage of a certain quantity of gasoline. It serves two purposes, namely, to keep all the other circuits of the carburetor supplied with the amount of fuel they need and to absorb the pulsation of the fuel pump, as it delivers the gasoline to the carburetor.
FLOAT SYSTEM
PURPOSE Efficient operation of the engine is dependent upon receiving the correct amount of fuel under all operating conditions. Therefore, it is the purpose of the float system to store fuel in the carburetor bowl and maintain the fuel at a specified or required level. This is accomplished by means of a movable float which will be raised or lowered depending upon fuel level moving the needle valve into or out of the need le seat. This float action, controlling needle valve movement, permits fuel to enter or be stopped according to float height.
FUNCTION
Float Action: When the engine is cranked or started, fuel is drawn from the gasoline tank by the fuel pump and forced or pumped under pressure to ent er the carburetor at the fuel inlet. When the level of the fuel in the carburetor float bowl is low, the float will assume a downward position allowi ng the weight of the needle valve to move it away from the orifice h ole in the needle seat. The force of the incoming fuel will also a ssist in moving the needle valve downward. Some carburetors will employ the use of a "pul l down clip," which connects the needle valve to the movable float arm, resulting in an immediate positive opening of the needle valv e as the float is lowered. This action prevents any possi bility of a momentary lowering of the fuel level due to a dela yed opening of the needle valve. Fuel will then enter the float bowl through the open ne edle and seat assembly. As the level of the fuel rises in the carburetor bowl, the float will rise and in turn force the needle valve into th e needle seat. When a pre de ter mine d le vel has bee n reach ed (d ete rmin ed by th e fl oa t le vel adjustment), the tip of the needle valve will contact the needle seat closing the orifice hole preventing any more fuel from entering t he carburetor bowl. As engine demands consume more fuel, the float will again be lowered allowing th e needle valve to move off its seat, repeating the cycle. Float Drop: A float drop tang located at the rear of the float a rm prevents the float from moving too far downward. The maximum float drop must be maintained so that the float assembly will drop sufficiently to allow maximum fuel flow into the carburetor under heavy engine fuel de mands. Bowl Vents: Some float systems will employ a fixed external vent as well as an
internal vent. We have shown in figure 10 both the in ternal and the fixed external vent. The external vent, located in t he air horn section above the float chamber, provides a means for venting to the atmosphere any fuel vapors or pressure that might accumulate or build up inside the float bow l ch amber du e to hi gh under -h oo d te mpera tu re s. If th es e vap or pre ssures are not di sp os ed of, th en the pre ssure in si de the bowl pu shin g down on top of the level of the fuel will be grea ter than the calibration of the carburetor intended it to be. This will result in a rich mixture due to excessive fuel being forced through the jets into the carburetor an d on into the intake manifold.
Balance Tube: All carburetors will employ an internal vent which is in a sense a bal ance tube balancing the pressure on top of the fuel with the p ressure of the a ir as it is entering the carburetor. By use of the internal vent, we are able to maintain this balance between the pressure pushing down on top o f the fuel in the float bowl with the pressure of the air as it is drawn into the carburetor. This action will co mpensate for a dirty or restricted air cleaner. Without this connecting or internal vent tube it is conceivable that, at sea level, there could b e a pressure of 14.7 lbs. per square inch pushing do wn on to p of th e fu el in the fl oat bo wl , but on ly a pr es s ur e of 13 lbs. (arbitrary figure) entering the carburetor due to a dirty or restricted air cleaner. Through this unbalanced arrangement we would have a rich carburetor mixture thereby upsetting the calibratio n of the carburetor. Idle Vent Valve: Most carburetors today will use a movable atmospheric idle vent valv e in pla ce of th e fi xed ex te rn al vent syst em. This movab le va lve wi ll be hel d in an open position only during idle and park positions by carburetor linkage contacting the idle vent arm. Any tendency for fuel vapors to collect inside the float bowl will be vented to atmosphere through this opening at idle and park positions. As car speed increases and t hrottle linkage is progressively opened, the actuating lev er from the linkage will no longer contact the idle vent ar m thereby allowing the idle vent valve to close. This action will now return the carburetor to the internal vent system thereby once again maintaining the same pressure on top of the
fuel as entering the carburetor through the air cleaner. It is important that the idle vent valve be closed d uring all periods of operation except at idle, otherwise excessive richness can be cau sed by the higher atmospheric pressure acting upon the fuel in the float b owl. Built-In Fuel Filter: From the opening and closing action of the needle a nd seat asembly, we can see why it is so important that the fuel be filtered and al l iron oxide or dirt particles be removed. Some carburetors are equipped with an integral fuel inlet filter located behind the fuel inlet fitting. On these models, the bronze filter element is spring loaded. This feature provides a pressure relief so that in the event the filter becomes clogged, th e restriction will cause fuel pump pressure to overcome the tension of the filter relief spring and allow fuel to enter by -passing the filter. In actual operation the float assumes a position that will allow the n eedle valve to open just far enough to replace the fuel at t he same rate at which it flows from the fuel bowl into the engine. Unde r idle or low-speed operation when very little fuel is needed, the nee dle valve will assume a slightly open position allowing only a small amount of fuel to ente r. If the incoming fuel is contaminated with dirt particles which could collect at the needle tip prevent ing the tip from assuming a close enough posit io n in to the seat, it is pos sibl e fo r more fu el to en ter th an is re qu ir ed by th e en gin e. As th e lev el of th e fu el ri se s in th e fl oat bo wl, th e di rt trapped between the needle tip and the seat will hold the needle valve open against the pressure of the float. The fuel would then rise excessively high in the float bowl, eventually running over the t op of the high-speed jet and on into the engine. We would n ow have a condition known as carburetor flooding. The excess fuel being literally dumped into the manifold would load up the engine to the point where the en gine would actually stall at idle and low speeds. Since the fuel from the fuel pump could still be under pre ssure it wi ll co nt in ue to en te r th e ca rb ur et or bo wl , du mpin g more fu el into the manifold making it extremely hard for the engine to be restarted.
ADVANTAGE OF VITON NEEDLE VALVE: As a means of preventing this flooding condition, a Viton-tipped needle valve is available and widely used. The Viton tip being soft will surround small particles of dirt allowing the needle valve to clo se. Without this resiliency and dirt-absorbing action a flooding con dition would result. As the fuel is consumed from the fuel bowl, the float drops, and the need le valve is moved off its seat. The incoming fuel which is under pressure from the fuel pump will now wash or carry away this small particle of trapped dirt. The ability of the Viton -tipped needle valve to absorb or digest small particles of dirt without encountering a flooding condition accounts for its wide popularity over the conventional steel needle and seat assembly. Other manufacturers may design their carburetors with the float assembly attached to either the air horn section (upper portion of carburetor) or the main body or fuel bowl section (center portion of carburetor). Regardless of design, they will all function in the same manner as described above .
IDLE SYSTEM
PURPOSE The purpose of the idle system is to provide the proper air/fuel mixture ratio to the engine at idle speeds and up to approximately 20 to 25 miles per hour. The position of the throttle valves determines how much air and fuel can be drawn into the engine to control engine speed. When the throttle valves are slightly open, as they are during engine idle, the vacuum or low pressure created from the downward movement of the pistons is confined to the area under the throttle valves. The idle discharge holes, being in this area of low pressure, will discharge fuel as long as the throttle valves remain in this approximate position. It will be noted that no fuel is flowing through the high-speed circuit. This is because air flow through the carburetor venturi is not great enough to produce a low-pressure area to cause fuel to flow from the main discharge nozzles. FUNCTION Each bore of the carburetor has a separate idle system. They consist of idle tubes, idle passages, idle air bleeds, idle mixture screws and the discharge holes. While
the following will describe what happens to cause the fuel mixture to follow the passage of the idle circuit feeding fuel to one bore, the action in the other half of the carburetor is identical. Because of the lower pressure area being maintained at the idle discharge hole, in relation to the higher pressure in the float bowl, fuel will flow from the float bowl through the metering jet into the main well area. Notice that the base of the idle tube has a calibrated opening or restriction which meters the flow of fuel entering the idle system. The fuel will continue fl owing up the idle tube to the top of the passage, in the venturi cluster, where it will meet the first idle air bleed. At this point a given quantity of air is permitted to be drawn in through the idle air bleed hole where it is mixed with the fuel. As the air and fuel meet at this point, the fuel becomes slightly lighter and is carried across the cross channel where it meets a second idle air bleed. More air is drawn in at this point and again mixed with the fuel aiding further in the atomization process. This air serves to break up the molecules of the fuel, making it less dense and easier to be burned. Calibrated Restriction: The fuel and air will now flow down the idle passage to a point where it will meet the calibrated idle restriction. The purpose of the idle restriction is to speed up the flow of fuel past this point while inducing a turbulent effect upon the fuel. This is accomplished in that a given quantity of fuel is flowing through the passage and as it meets this restriction must speed up at this point in order to maintain the volume of fuel flow. This physical change of fuel from a heavier to a lighter consistency is necessary in order for the fuel to be more easily ignited and burned inside the engine cylinders. Idle Discharge Holes: As the fuel flow continues downward, it now meets two secondary idle discharge holes. These secondary idle discharge holes serve as a third idle air bleed when the throttle valves are in the present position. Since the low-pressure area is confined to the side and just bene ath the throttle valves and not on top, air will be drawn through the secondary idle discharge holes to further mix with the fuel aiding in the more complete atomization of the fuel. The fuel now reaches the idle discharge hole where it is then drawn into the carburetor bore by the low pressure being exerted at the discharge hole.
Idle Mixture Screw: The amount or quantity of fuel that can be released through the idle discharge hole is controlled by the position of the idle mixture screw. It is readily seen that by turning the screw inward or clockwise, the tapered end of the needle tip will reduce the amount of area at the idle discharge hole thereby reducing the amount of fuel which can get by, resulting in a leaner mixture. Rotating the idle mixture screw outward or counterclockwise will enlarge the size of the idle discharge hole, thereby permitting a larger quantity of fuel to flow by giving a richer mixture. As the fuel is drawn from the idle discharge hole, it is suddenly subjected to the rapid movement of the air stream which is the final step in the metering and transformation process of changing the fuel from a heavy dense liquid to a light atomized combustible mixture. From the action of the idle air bleed holes permitting a given quantity of air to enter and mix with the fuel, it is readily apparent that if dirt should block or seal the opening an extremely rich mixture will result causing the engine to idle rough. Also, when adjusting the idle mixture screw, it should be only lightly bottomed and never forced into its seat. If it is excessively tightened, the tapered tip will become grooved destroying its ability to arrive at or control the precise amount of fuel flow necessary for smooth idle.
PART THROTTLE OR MAIN METERING SYSTEM
PURPOSE The purpose of the part throttle or main metering system is to provide fuel during the transition period from off-idle to wide-open throttle operation. Under power operation, it is supplemented by the power system. It will be noted that the quantity of fuel being discharged from the idle and secondary idle discharge holes has diminished due to the wider throttle valve opening resulting in a decrease of intake manifold vacuum acting on these holes. FUNCTION As the throttle valve opening increases, fuel begins to flow through the main metering system due to an increase in air velocity through the venturi system. This causes a drop in pressure in the main or primary venturi which is increased many times in the secondary venturi. Because a low-pressure area is now acting at both points (main and idle circuits), fuel will flow in the following manner. Tracing the path of fuel, we see that the fuel flows from the float bowl through the main metering jet into the main well area. The size of the main metering jet determines how much fuel can flow through. The fuel will now divide: Some will flow through the idle circuit and some through the main metering circuit.
Idle Fuel Flow: The fuel flowing through the idle circuit will move up the idle tube to the top of the passage where it meets the first idle air bleed. Here air is drawn in and mixed with the fuel. This mixture travels across the cross channel where it is bled a second time by air from the second idle air bleed. From there the lighter mixture of air and fuel travels down the idle passage past the calibrated idle restriction and discharges into the bore of the carburetor through the idle and secondary idle discharge holes. Main Metering Fuel Flow: Fuel flows through the main metering system due to the low pressure in the secondary venturi being transmitted to the tip of the main well tube. Pressure within the fuel bowl which is greater than venturi pressure forces fuel into the main well area through the metering jet. The fuel then travels up through the center of the main well tube. Air entering through the main well air bleed will mix with fuel through the uncovered holes at the top of the main well tube. This action serves to break up or help atomize the fuel in the main discharge circuit the same way as the idle air bleed serves to break up fuel particles in the idle discharge passage. The mixture of air and fuel now continues up the main well tube to the top of the venturi cluster assembly. Because of the low pressure or suction acting on the fuel at this point, the fuel is made to flow down the mixture passage where it is further mixed with air being drawn through the open end at the top of the mixture passage. The fuel continues flowing to the secondary venturi and on into the intake manifold. It will be noted at the base of some secondary venturies a brass cage containing a series of small holes is sometimes used. This cage in effect also helps to break up liquid droplets of fuel changing them into a fine mist state for easier burning. With the increased throttle opening, there is an increase in the velocity of the air through the venturi system. This causes a further drop in pressure resulting in a larger quantity of fuel flow through the main metering system and a corresponding drop or lessening of fuel from the idle system to the point of complete stoppage. As the throttle opening is progressively increased causing more fuel to be drawn through the main well tubes, the fuel level in the main well drops. As this fuel level drops, more calibrated holes in the main well tubes become exposed. When this
occurs they become air bleeds, thus mixing progressively more air with the increase of fuel passing through the main well tubes. Thus, although the nozzle suction is increased by increasing the throttle opening, the fuel mixture to the engine remains constant throughout the part throttle range. If this action (lowering of the fuel level) in the main well did not occur, the mixture would be overly rich, resulting in a stumbling engine during the part throttle range.
POWER SYSTEM
PURPOSE The purpose of the power system is to provide the extra quantity of fuel required when more power is desired or extreme high speed driving is to be maintained. The Rochester 2-barrel carburetor uses a vacuumoperated power system. During the period of high-speed driving when fuel needs are extremely great, the amount of fuel which could normally enter the engine through the metering jet is not sufficient to satisfy engine demands. Therefore, other means of supplying additional fuel to the engine under these conditions must be used. This is accomplished by use of the power system.
FUNCTION Designed into the power system is a spring-loaded power piston and stem assembly located directly above a power valve assembly. During idle and normal cruising speeds when intake manifold vacuum is strong, this vacuum which is transmitted through a vacuum passage to the top of the power piston assembly will hold the power piston upward, keeping the stem of the power piston assembly away from the power valve assembly. The power valve is now in a closed position with no fuel permitted to flow through. The only fuel now reaching the engine is that which is flowing through the main metering jet. Under extreme power demands, intake manifold vacuum will weaken and drop to a point where the tension of the power piston spring will overcome the strength of the existing vacuum acting on the top of the power piston causing the power piston and stem to be pushed downward contacting the power valve assembly. The power valve assembly will now be held open allowing fuel to flow through a passage, containing a calibrated restriction, and on into the main well area supplementing the fuel which could normally enter the main well area through the metering jet. The combining action of fuel from these two places is enough to satisfy engine demands. The combination of fuel through the power valve assembly and the main metering jet will now flow up the main well tube into the mixture passage and be drawn into the area of high vacuum or suction at the secondary venturi. Idle Fuel Ceases: It will be noted that the fuel has discontinued flowing through the idle passage. This is because the throttle valves are now in a wide open
position and the low pressure area is no longer concentrated in the vicinity of the idle discharge holes. Due to the large volume of air rushing into the carburetor past the primary and secondary venturi, the low pressure or vacuum is now confined solely to this area. When a return-to-normal cruising speed is desired and the throttle is progressively closed, the extra fuel from the power system is no longer required. The vacuum in the intake manifold now begins to increase or build up once again. As the strength of the vacuum increases, it is again transmitted through the vacuum passage to the top of the power piston assembly. When the vacuum reaches a predetermined value, it will overcome the tension of the power piston spring, pulling the power piston and stem assembly upward allowing the power valve to once again close preventing any further flow of fuel. A small spring contained inside the power valve assembly keeps the valve in a closed position. Vacuum Bleed-Off Port: There is a hole or vacuum bleed off port drilled through the wall of the power piston cylinder. This vacuum bleed off port is necessary for the following reason: Because vacuum in the vacuum passage is acting on top of the power piston, if it were not for the bleed off port, it would be possible for the air in the float chamber to be drawn out past the power piston and cylinder wall. This removal of air would lower the air pressure in the float chamber seriously affecting the calibration of the carburetor. As previously stated, the carburetor operates under the theory of pressure differences. This means that fuel is made to flow according to the amount of pressure or vacuum acting on the exit hole of that particular passage in the carburetor. Therefore, the action of the vacuum bleed off port allows air to be drawn from the top section of the carburetor where no harm is done instead of allowing air to be withdrawn from the float chamber itself.
Additional Air Bleeds: Notice as the throttle valves are held in the wide open position, the level of fuel in the main well area has dropped to its lowest point. This action uncovers additional holes in the main well tube which then serves as additional air bleeds. Although the volume of fuel has increased, so has the volume of air,thereby maintaining a constant mixture ratio to the engine throughout this power range. Two-Step Power Valve: Some Rochester models use a "two-step" power valve assembly. The first step unseats the spring loaded plunger and fuel is metered by the plunger for light power requirements. For the second step, the plunger is completely bottomed and fuel is metered entirely for full power operation by the calibrated restriction in the passage leading to the main well area.
CHOKE SYSTEM
PURPOSE The purpose of the choke system is to provide an extremely rich fuel mixture for cold engine starting and operation. This rich mixture is required because fuel vapor has a tendency to condense into dro plets of raw gasoline upon coming into contact with the cold wall of the intake manifold. This action seriously robs the amount of fuel which normally would reach the engine cylinders to be ignited and burned. You undoubtedly have noticed t hat if you place a glass of water in a room how moisture will immediately begin to form on the outside of the glass. This is condensation being formed by the warm air coming into contact with the cool surface of the glass causing beads or droplets of water to be condensed from the moisture in the air collecting on the glass. This same process takes place inside a cold engine. FUNCTION Whenever a cold engine is cranked with the choke valve closed, the vacuum which is created inside the intake manifold by the downward movement of the pistons will be transmitted inside the carburetor to the underside of the choke valve. By having this low pressure area confined within the entire bore of the carburetor, atmospheric pressure in the fuel bowl will force fuel to flow from both the high-
speed nozzles as veil as the idle discharge holes. This action furnishes the necessary rich mixture. Choke valves may be either manually or automatically operated. In the Rochester 2-barrel line, the model 2G used mostly on trucks and marine engines has a manually operated choke. Models 2GC and 2GV, used mostly on passenger cars, have an automatically operated choke. Model 2GC means a thermostatic coil and housing assembly operated by hot air will be mounted on the carburetors at either the air horn or throttle body sections. On units with the housing attached to the throttle body (some models of Buick, Oldsmobile and Pontiac), an intermediate choke rod adjustment is necessary. Principles of operation are the same regardless of housing location. Full adjustment instructions and procedures are included in Chapter IV as well as the individual instruction sheet supplied in every Echlin carburetor tune-up kit. Model 2GV means the thermostatic housing is not an integral part of the carburetor assembly. Instead the thermostatic coil is located in a well in the exhaust manifold and is connected to the choke valve by means of a rod. Most automatic choke carburetors today, regardless of the manufacturer, will be of either type listed above. While the following description of operation pertains to the two types of Rochester choke systems, the general theory will apply to all carburetors in use today.
ROCHESTER MODEL 2GC CHOKE SYSTEM:
This choke system consists of a thermostatic coil assembly, off-set choke valve, choke piston, fast idle cam and linkage. The thermos coil is calibrated to hold the choke valve closed when the engine is cold. Because the choke valve is mounted off-center, as the engine starts air velocity against the larger off-set portion causes it to open slightly against the torque of the thermostatic coil spring. Also, intake manifold vacuum which is acting on the choke piston through a vacuum passage tends to open the choke valve. After the initial start, the choke valve assumes a position where the torque of the thermostatic coil spring is balanced against both the vacuum pull on the choke piston and air velocity against the off-set choke valve. As the engine warms up, hot air heated by the exhaust manifold is pulled into the thermostatic coil housing. The coil spring, when heated, gradually relaxes its tension allowing the vacuum pull on the choke piston and the air velocity through the carburetor to fully open the choke valve. Choke Piston Action: The choke piston tends to modify choke valve action compensating for varying engine loads upon accelerating a cool engine. Upon acceleration or increased engine load, manifold vacuum decreases lessening the pull upon the choke piston. This permits the thermostatic coil spring to momentarily increase choke valve closure providing the engine with a slightly richer mixture for acceleration.
Fast Idle Cam Action: As a means of preventing engine stalling during the warm-up period, the carburetor linkage is designed to increase engine speed whenever the choke valve is in a partially closed position. This is accomplished by use of a fast idle cam which is linked to the choke valve shaft. The cam, which has steps of varying height, contacts a screw attached to the throttle shaf t. The rotation of the fast idl e cam in relation to choke valve opening holds the throttle valves open sufficiently during warm up to increase the idle speed. The greater the choke valve closure, the faster the engine will idle. As the choke valve moves to wide-open position, the idle speed then returns to normal setting. Choke Unloader Action: During the warm-up period, the vehicle operator may wish to advance the throttle to wide-open position. Because manifold vacuum now decreases, this would decrease the pull upon the choke piston thereby closing the choke valve, overchoking the engine. As a means of opening the choke valve mechanically to provide for increased carburetor air flow, the throttle lever is made to contact a tang on the fast idle cam. Revolving the fast idle cam results in the positive opening of the choke valve. This choke unloading action also serves to de-choke a flooded engine during starting, whenever the engine is cranked with the accelerator held fully depressed.
Rochester Model 2GV Choke System:
This type choke system also uses the off-set choke valve, fast idle cam and linkage but will have the thermostatic coil located on the exhaust manifold and connected to the choke valve by a rod. This location permits the thermostatic coil to sense engine heat from the exhaust manifold, thus controlling choke valve position - in the same manner - as air heated by the exhaust manifold and brought to the thermostatic housing, in the 2GC system, permits the thermostatic coil to sense engine heat. Because the metal choke piston in the 2GC system is prone to sticking due to carbon accumulation, a vacuum diaphragm uni t mounted on the carburetor air horn and linked to the choke shaft is used in place of t he choke piston. When the engine is started manifold vacuum acting on the diaphragm opens the choke valve to the point where the engine runs without loading or stalling thus performing the same function as the choke piston. The operation of the fast idle cam and linkage is identical to the 2GC system. Carburetors of all makes and models having an automatic choke will use either the hot air system (2GC type) or the heat- sensing manifold unit system (20. type). Theory of operation will be the same as previously described.
