Float-Bowl Ventilation
The float bowl,above the level of the fuel,must have a vent to the atmosphere.atmospheric pressure is necessary over the fuel to provide the high pressure necessary to push the fuel into the venturi and other low-pressure areas.most carburetors have two types of vents:internal and external.
  The internal vent or balance tube opens the fuel bowl to the air from an area of the  carburetor above to the choke valve.the function of this vent is to equalize the effects of a clogged or restriced air cleaner.for instance,suppose that some dust and have grit have clogged the air cleaner element;this restricts the air flow through the element.as a result,a the carburetor,which adds to the vacuum developed by the venturi.this action causes excessive fuel flow from the nozzle.
   However,the internal vent also applies this same partial throat vacuum to the fuel bowl.Consequently,the atmospheric pressure in the bowl pushes some sir through the vent to fill this void or vacuum.this action results in a reduction in air pressure above the fuel level in the bowl.Therefore,the fuel level flow from the nozzle is more normal because the internal vent has balanced the air pressures between the top of the carburetor throat and the fuel bowl.
   The external vent,as its name imp;ies,permits atmospheric pressure outside the carburetor to enter the fuel bowl.But at the same time,it allows gasoline vapors to escape from the fuel bowl.This adds harmful pollution to the atmosphere.However,an external carburetor vent is necessary to prevent fuel percolation.Percolation is the build up of vapor pressure due to heat expansion or boiling of the fuel in the bowl as a result of heat radiation from the engine.This vapor pressure can force raw fuel from the bowl through the fuel nozzle and into the intake manifold.the result is a flooded engine and an empty fuel bowl,either of which causes hard stsrting.
  In some older carburetors,an antipercolator valve controlled the opening of the external vent.When the engine was shut off or idling,carburetor linkage opened the valve.This allowed fuel vapors to escape to the atmosphere.As a result,there were fewer vapors entering the intake manifold,and the engine would start and manifold,and the engine would  start and idle better.Finally,during zbove-idle operating speeds,carburetor linkage closed the valve,and the internal vent took over the task of supplying air pressure to the float bowl.
  Since 1970,the majority of automotive carburetor external vents connect into a vapor control system.This system still premits the admittance of atmospheric pressure over the fuel level and prevents fuel percolation by allowing vapor pressure to escape from the bowl.But in the latter case,the vapor system stops the hydrocarbons from reaching the atmosphere.This texe covers this system in detail in another chapter.

Operation
  In the idle position,the throttle valve is only slightly open.This permitd only a small amount of air to pass between the wall of the carburetor throat and the edge of the throttle valve.Since at this point there is insufficient air floe for vevturi zction,fuel floe within the idle system is the result of intake manifold vancuum acting dirctly on the idle-discharge port,through the idle system ,and on the fuel in the fuel bowl.
  This  low pressure below the throttle valve and fuel atmospheric pressure above the level of the fuel in the bowl force the liquid gasoline through the idle tube and into the idle passage.Near the top right side of the passage,the fuel mixes with air from the top air bleed.This action constitues the first stage of fuel atomization.The mixture then continues down the passage,past the off-idle port,where the mixture picks up additional air that further breaks up the mixture.The mixture finally flows past the tip of the mixture screw and sprays into the carburetor throat,where the passing air carries it into the intake manifold.
  As the drive depresses the accelerator pedal past the idle position,the throttle valve opens;and addittional air flows through the carburetor.Since the air flow at this point is still insufficient to cause a fuel discharge from the venturi nozzle, the increased air velocity results in an excessively lean mixture. Tcompensate for this problem, atomized fuel must begin to discharge from the off-idle or low-speed port (Fit. 8-25).
  Moving the throttle slightly open, past the idle position, exposes the off-idle discharge ports to intake manifold vacuum. At this point, the ports to intake manfold vacuum. At this piont, the ports stop acting as additional air bleeds and begin to discharge atomized fuel. Thus, the off-idle or lowspeed port openings have a dual purpose. At idle they act as an air bleed; but during low-speed operation, these ports discharge an atomized fuel mixture into the passing air flow.
  Both the idle and off-idle ports are necessary to provide the smooth transition between engine idle and cruising speed. Depending on carburetor design, this cruising speed may be between 25 to 40 mph for passenger automobiles. At these approximate vehicle speeds, the throttle opening and resulting air flow are great enough to permit the main metering system to begin functioning.
  The point at which atomized fule begins to flow the main nozzle is the transfer point shown in Fig. 8-26. At this point, the carburetor is passing from the idle to the main-metering system. However,fuel discharge, from the idle and off-idle ports, dose not completely cease at this particular point but rather diminishes. As a result, the two systems interact to produce a very smooth air/fuel flow during these vehicle speeds.

Mdin-Metering System
  The main-metering system is responsible for supplying an air/fuel mixture for high-speed engine performance,the negine is not under heavy loads and therefor can operate on an economically lean air/fuel mixture.The carburetor components of this system are the main jet,main well,main nozzle,and the air bleed.
  The main jet fits between the fuel bowl and the main well.This main jet is a very accurately machined orifice(restriction) that controls the queantity of liquid fuel flow from the fuel bowl into the main well.In some carburetors,the size of the opening in the jet alone controls the fuel flow rate.However,in the carburetor shown in Fig.8-28,a tapered metering rod passes through the opening in the jet.In this installation,the rod,by being inside the jet opening,provides a restriction to fuel flow.The movement of this metering rod up or down alters the jet restriction and therefoce changes the amount of fuel flow.
  The main well forms the working fuel resercoir for the main-metering system.Tuel entering this well must first pass through the main jet.Consequently,the main jet controls the quantity of usable fuel in the well when the main system is in operation.The result is that only a givven,maximum amount of fuel is available in the main well and therefore is the controling factor regarding the total fuel available for delivery by the main nozzle.
  The main nozzle discharges an atomized air/fuel mixture into the throat of the carburetor.The discharge tip of this nozzle is located at the point in the venturi of greatest restriction.The opposite end of the nozzle fits into the main well.With this design,the fuel only travels a short distance between the well and the tip of the nozzle.
  The air bleed adds air to the liquid fuel to atomize it partially.In the system shown in Fig.8-27,the air bleed is the calibrated opening in the nozzle,slightly above the main well.Atmospheric pressure forces air through this opening anytime there is a decarease in pressure at the nozzle tip.
Operation
  As the driver opens the throttle to a point where the main-metering system begins to function-the transfer point shown in Fig.8-26-air velocity through the ventui increases,which in turn decreases the pressure in the venturi at the tip of the main fuel nozzle.As a result,atmospheric pressure over the fuel in the bowl pushes additional liquid fuel through the main jet and into the main well,where it rises in the main well pssage leading to the nozzle
  Then,the liquid fuel begins to enter the main nozzle,where it mixes with air coming through a calibrated hole in the lower portion of the nozzle.The air assists in bresking up the liquid fuel for improved distribution and total atomization later as the fuel leaves the nozzle.
  The partially atomized fuel then continues through the nozzle until it sprays from its tips.The atomized mixture then enters the air stream at the boost venturi.At this point,the air/fuel mixture mixes with the incoming air that carries it past the throttle vavle and into the intake manifold for distribution to the engine cylinders.
  The quantity of the air/fuel mixture flow and the resulting engine speed aer factors determined by air velocity and jet size.For enample,to alter engine speed,additional air/fuel floe is necessary.This means that the throttle valve must be open further,which in turn permits more air flow through venturi.As a result,there is a greater reduction in pressure at the tip of the fuel nozzle in the centuri and greater fuel flow.Consequently,the en1ine accelerates.
  However,there is a limit to how far the engine can accelerate on this system.This limitation is due to the size of the main jet opening.In other words,the jet limitsthe liquid fuel flow into the system.By so doatomized mixture exiting from the nozzle tip.When the engine reaches its maximum,it can no longer accelerate.
Power system
Function
   The main-metering system provides the leanest air/fuel mixture of any of the other carburetor circuits;a richer mixture is necessary not only for extended high-speed operation but also for maximum engine power.For maximum engine power,a rich fuel mixture is mandatory in order for the combustion process to consume all the oxygen in the air entering the combustion chamber.To accomplish this action,the carburetor has some type of power system used to supplement the main-metering system.This power system provides an increase in fuel mixture flow from the nozzle tip,according to the amount of throttle opening and engine load.In other words this additional system function to enrich the total air/fuel mixture during any phase of main-metering system operation,depending on throttle position and engine-load requirements.
Design
  The power system of the carburetor shown in Fig.8-28 consists of the main-metering system,metering rod,power pisto ,and power-piston spring.The metering rod is a restriction placed into the opening of the main jet,thereby reducing the amount of fuel flow through it.The rod provides the greatest restriction during light load or the economy phase of operation of the main-metering system.When extra fuel flow is necessary for full power,the rod moves upward in the jet to decrease the restriction and increase the fuel flow.
  Since the rod does not completely move out of the jet,it has a special design feature that gradually increases fuel flow through the jet as the rod moves upward. This feature is in the from of a stepped or tapered end that,when moved to various position in the jet,increases or decreases the jet restriction.Finally,the metering rod activates either through mechanical linkage connected to the throttle linkage or through a power piston.
   The power piston connects through a special piece of linkage to the end of the metering rod.The power piston is responsible for lowest position in the main jet.To accomplish this function,the piston itself moves within a special bore in the carburetor housing.This bore has a passage that supplies intake mainfold vacuum to the bore and power piston pulled down in its bore and therefore maintains the attached metering rod in its lowest position  in the main jet.
  Under the power piston is a calibrated power piston spring.The function of this spring is to raise or push up the power piston under low-vacuum conditions.This action,of course,moves the metering rod upward.
  The actual tension of this spring varies among the various carburetors according to the size of the power piston and under what vacuum cconditions the manufacturer desires the metering rod to move upward.For example,the spring of a typical carburetor has a calibrated tension that allows the piston to begin its upward movement at a vacuum of 8 to 9 inches Hg.Full upward movement of the piston and metering rod occurs at 4 to 6 inches Hg.
Operation
  When the engine is operating under light load or moderate speed conditions,engine vacuum is high.In this situation,the high vacuum holds the power piston all the way down in its bore.This action maintains the largest diameter area on the end of the metering rod inside the main jet opening.The metering rod now sufficently restricts fuel flow through the jet for the normal lean operation of the main-metering system.
  However,when there is a demand for greater speed or engine power,the driver must open the throttle valve.As this valve opens,engine vacuum drops rapidly.With a reduction in vacuum acting on the bottom of the power piston,the spring begins to move it upward in its bore.This also forces the metering rod upward so that a smaller area of the rod is inside the jet.As a result,there is an increase in fuel flow through the the main jet to the main nozzle.
   While this particular system is always operational at vehicle speeds and loads rpquring wide-open throttle,sudden throttle openings at slow and mid-range engine rpm also cause the momentary lifting of the metering rod due to the decrease in manifold vacuum.The main two factors to remember about this system aer that its only function is to produce an enriched mixture while the main metering system is in operation and the actual drop in engine vacuum determines just how much of the metering rod moves out of the jet.
  Finally,there is another style of power system on many carburetor types.This system dose the same thing as the one just described;however,it uses a separate power valve instead of metering rods.The power valve in this system,when open,bypasses fuel around the main jet to enrich the main-metering system.To control the operation of this power valve,manufacturers use engine vacuum,which acts against either a diaphragm and spring or a piston and spring.In either case,engine vacuum cloes the valve and the spring opens it.

Pump System
Function
  When the driver opens then throttle valve rapidly, from a closed or nearly close position, in order to accelerate the vehicle, engine vacuum rapidly drops; but the air flow through the carburetor throat increases instantly. Due to the great difference in weight between air and fuel, the flow of fuel from the carburetor circuits lags behind the increase in air intake. As a result, the engine experiences a momentary leanness, which causes a brief engine hesitation, stumble, or flat spot.
  The pump system provides the additional fuel flow necessary to overcome this leanness and maintains smooth engine operation during rapid low-speed acceleration. To accomplish this task, the system discharges additional fuel into the venture air stream whenever the throttle valve initially opens. However, this system does not function beyond the point where the throttle valve is approximately half open.
Design
  A typical pump system, shown in Fig. 8-30, consists of a pump plunger, duration spring, return spring, inlet valve, outlet valve, and pump jet. At its upper end, the pump plunger connects to a pump lever that fastens to a link attached to the throttle valve shaft. When the throttle valve opens, this link pulls the pump lever down, which in turn also causes the pump plunger to move downward into its bore. Conversely, if the throttle closes, the link pushes the lever and its attached plunger upward. This action pulls the pump plunger upward.
  On the opposite end of the plunger from the lever is the pump cup or seal. This cup forms a piston that applies the force to the fuel necessary to push it through the pump passages and past the pump jet. Also, as it moves upward, the piton cup creates a slight difference in pressure that brings liquid fuel from the fuel bowl into the pump’s bore or well.
  The duration spring also moves the plunger and cup downward in the well as the throttle opens. By using a calibrated spring for this purpose, along with a mechanical connection, the manufacturer controls the duration of the pump’s discharge. Then is a necessary consideration if or when the driver quickly opens the throttle valve in order to prevent the system from discharging all its fuel too rapidly, which can cause engine stumble during rapid engine acceleration.
  The duration spring itself fits over the plunger, between a flange and washer above the piston cup and a curved washer bearing against a shoulder on the plunger shaft. With this arrangement, any lengthy upward movement of the plunger, such as when the throttle valve closes causes the curved washer to bear against the top of the carburetor casting. As upward plunger movement continues, the duration spring compresses between the curved washer and the piton cup.
  As the driver opens the throttle valve, the pump link begins to move the plunger down, and the duration spring begins to expand. This action pushes the piston cup downward in the pump well. If the well contains fuel, the pump cup applies force to it and begins pushing it through the pump the system passages. The spring continues to expand and farce the plunger downward until the cup reaches the bottom of the well. The spring action during this time lengthens the time period for plunger travel in the pump well and therefore lengthens the duration of the fuel discharge from the system.
  Also, there , must be some from of mechanism built into the plunger, duration spring, or linkage arrangement for additional throttle valve movement, once the plunger bottoms in the well. In the carburetor shown in Fig. 8-30, this mechanism consists of a plunger made in two sections, which cannot separate but telescope over one another. The tension of the duration spring keeps both plunger sections extended or apart, which provides normal plunger length during the intake phase and the beginning of the discharge phase of pump operation.
  However, once the plunger cup bottoms in the well, the upper plunger section continues to move downward with the further travel of the throttle valve and linkage. As a result, the upper section telescopes over the lower section, which compresses the duration spring. In other carburetor styles, manufacturers achieve the same result by either slotting the end of the plunger where it attaches to the pump lever or permitting the end of the plunger to telescope inside the pump lever.
  The carburetor illustrated in Fig. 8-30 has a pump return spring; however, some carburetor do not have this device. When used, this spring assists the mechanical linkage in moving the plunger to the up position as the throttle closes.
  The inlet valve fits between the main fuel bowl and the pump well. Its function is to permit fuel to flow from the bowl to the well during the pump’s inlet stroke. But this one-way valve prevents a backflow of fuel from the pump well to the fuel bowl during the pump stroke. This valve may be in the from of a metal ball located in the from the well to the bowl or be part of the plunger assembly. In this latter case, the cup itself moves up o r down on the plunger head to from a valve. When the plunger moves upward, the flat area on top of the cup unsets from the flat on the plunger head. This action allows free movement of fuel, fed into the well above the cup, through the inside of the cup to the bottom of the well, however, as the plunger moves down, the cup moves upward, trapping the fuel in the well.
  The pump discharge or outlet ball valve is also a one-way check valve. This device permits fuel to move from the pump well, through the discharge passage, and out the pump jet during the pump’s delivery stroke. During the inlet stroke of the pump, the spring holds the valve closed. This action prevents any air from entering the discharge passage, which would reduce the efficiency of the plunger cup in drawing fuel into the well.
  The pump jet is nothing more than a calibrated opening in the wall of the carburetor throat. In the carburetor pictured in Fig. 8-30, the jet opening is above the entrance to the primary venture. With this location, the jet discharges fuel into the air flow between the primary and boost venturies.
Operation
  Whenever the throttle valve closes, the plunger moves upward in the pump well, creating a slight difference in pressure between the well area below the plunger cup and the atmospheric pressure above the fuel in the float chamber or bowl. As a result, fuel from the float bowl enters the pump well through an inlet check valve or through the slot in the top of the pump well (Fig. 8-30). In the latter case, the fuel then flows past the pump cup seal and into the bottom of the pump well. At the same time, the discharge check valve seats in order to prevent air from leaking into the system.
  When the driver opens the throttle valve, its connecting linkage along with the duration spring begins to force the plunger downward. The pump cup seats itself against the plunger head and begins to force fuel through the pump discharge passage. By seating the pump cup or seating the inlet check valve, if so equipped, the pressurized fuel cannot return to the float bowl. Instead, the fuel passage, through the open outlet valve to the pump jet, where it sprays into the venture area.
  Although the driver may immediately move the throttle valve wide open, the plunger does not bottom in the pump well instantly. Because this plunger has two sections that telescope over one another, the resistance of the fuel flow to movement within the pump passages forces the lower section momentarily to stop its travel while the upper section continues to move down over it. Then, the duration spring continues to move the lower plunger section and the cup down. This action permits fuel delivery by the pump plunger cup for a short period of time after the throttle linkage movement ends.
  During high-speed engine operation, the vacuum formed at the pump nozzle in the carburetor throat may be sufficient to unseat the outlet check valve and siphon fuel from the pump system. In some carburetors, this additional fuel is part of the normal main-metering system air/fuel mixture calibration. However, in other carburetors it is not, so this fuel siphoning creates an overly rich mixture at higher speeds.
  To stop this siphoning effect, manufacturers can modify the pump system in one of three ways. First, the system may have an air bleed machined to the discharge passage. Second, the weight of the discharge check valve or the tension of its spring may be increased. Finally, the pump plunger itself may contain some form of antisiphon check valve.
Choke System
  When starting a cold engine, the factors necessary for good fuel vaporization are missing or inadequate. For this reason, it is necessary to provide extremely rich mixtures from the carburetor, 2:1 to 1:1, in order to provide sufficient combustible mixtures to all the cylinders for quick staring. The carburetor obtains this enrichment by the addition of a choke valve in the carburetor throat above the venture and main nozzle. This choke valve, during cold engine starting, starts fuel to flow through the main system prematurely.

Design
  The choke illustrated in Fig. 8-31 consists of a choke valve, vacuum break assembly, fast-idle cam, choke un-loader, and a thermostatic coil. The choke valve is an offset plate, which pivots on a shaft located near the top of the carburetor throat. The function of this choke valve is to restrict the normal air flow through the carburetor throat. By doing so, intake manifold vacuum imposes its effects on the idle as well as the main-metering system. The result is, of course, an extremely rich mixture.
  The vacuum break assembly contains a diaphragm that attaches through linkage to the choke valve. When the engine is running, this diaphragm has intake manifold vacuum applied to it on one side and atmospheric pressure on the other. This causes the diaphragm to move and open the choke valve partially. This action prevents the started engine from stalling due to an overly rich mixture. Note: on other carburetor styles, this device has a vacuum piston instead of a diaphragm, and the entire unit is contained in the same housing as the bimetallic coil (Fig. 8-32).
  During engine warm-up, it is very necessary to increase engine idle rpm to prevent the engine from stalling; this is the function of the fast-idle cam (Fig. 8-33). This cam connects by a rod to a lever on the choke valve shaft. The cam itself has graduated steps upon which the idle screw or tang on the throttle lever contacts. As the choke closes, the lever rotates the cam so that its largest step is controlling idle speed that, in this case, in much higher than the normal hot idle speed. When the engine is fully warm and the choke valve is wide open, the fast idle cam rotates so that the idle screw rests on the low step on the cam. Then, the engine operates at normal curb idle. Finally, from the point where the choke valve is fully closed until it is wide open, the cam rotates gradually, allowing one step at a time to engage the idle screw. This action permits a gradual reduction in idle rpm as the engine warms up.
  A choke unloader mechanism is necessary if the engine becomes flooded during cold starting (Fig. 8-33). This device partially opens the choke valve to increase the air flow through the carburetor when the driver depresses the accelerator pedal to the floor. The extra air leans out the fuel mixture sufficiently so that the engine will start.
  The unloader mechanism consists of a projection on the throttle lever and the fast idle cam. When the driver depresses the gas pedal down all the way, the throttle-lever projection contacts the edge of the fast idle cam. This action forces the cam to rotate, which in turn forces the choke lever rod to turn the choke lever and shaft; and the choke valve partially opens.
The thermostatic coil assembly may be a remote until attached to a heated well on the intake manifold (Fig. 8-31) or inside a round housing on the carburetor housing near the choke valve. In the latter case, this bimetallic coil receives its heat from the exhaust manifold or from hot coolant from the cooling system; and the housing also contains the vacuum break piston (Fig. 8-32). Regardless of the location, the bimetallic coil is responsible for closing the choke when the engine is cold.
Operation
  A combination of intake manifold vacuum, off-set choke valve, action of the bimetallic coil, atmospheric temperature, and exhaust manifold heat controls the operation of the choke system shown in Fig. 8-31. For example, before the driver starts a cold engine, atmospheric temperature causes the bimetallic coil to wind up; this action through its control rod closes the choke vale. With the choke valve closed, the carburetor supplies a very rich mixture to the engine combustion chambers as the engine turns over for starting.
  Once the cold engine starts, two factors initially cause the choke valve to open slightly to prevent the engine from stalling. First, because the choke valve is offset in its support shaft, air velocity causes the valve to open slightly against the torque of the thermostatic (bimetallic) coil. Second, intake manifold vacuum, applied to the diaphragm or piston of the brake unit, pills the choke valve open to a given position until the engine begins to warm up.
  As the engine warms up, the application of exhaust manifold heat causes the bimetallic coil to relax or unwind. This action gradually decreases the torque on the choke valve until it is fully open. The full opening of the choke valve also is the end result of two factors; heat and air velocity. Manifold heat on the coil causes it to unwind, releasing its hold on the choke a remote unit attached to a heated well on the intake manifold (Fig. 8-31) or inside a round housing on the carburetor housing near the choke valve. In the latter case, this bimetallic coil receives its heat from the exhaust manifold or from hot coolant from the cooling system; and the housing also contains the vacuum break piston (Fig. 8-32). Regardless of the location, the bimetallic coil is responsible for closing the choke when the engine is cold.
Operation
  A combination of intake manifold vacuum, off-set choke valve, action of the bimetallic coil, atmospheric temperature, and exhaust manifold heat controls the valve. With on torque to hold the valve toward the closed position, the air flow past the offset choke valve forces it fully open.
  If the driver accelerates the engine during the warm-up period, intake manifold vacuum drops. As a result, the vacuum brake become temporarily inoperative and closes the choke valve slightly. The amount of closure depends upon the amount of air velocity past the choke valve and how far the choke coil has relaxed. In any case, the result is a richer mixture for acceleration during the warm-up period.