Carburetor Operation



The fundamental task of carburetors is to supply vaporized fuel to an internal combustion engine.  Vaporized fuel is created when air is drawn through the carburetor and mixed with fuel using the fluid dynamics of the in-rushing air.  Fuel is drawn from the fuel well (at a metered rate) and mixes with the air stream to create a mixture of air and fuel in a form that may be burned quickly and completely by the engine.  Carburetors are rather good at this process considering there aren’t any computers involved but they only perform as well as a tuner can adjust them.  Complete combustion requires the air/fuel mixture to be vaporized without liquid droplets, which won’t burn. 

Fuel is supplied to the carburetor and held at a pre-set level in the two fuel wells in each throttle body.  The fuel is maintained at a constant level using floats within the fuel wells acting to open and close the needle valves the fuel is supplied through.  As fuel is drawn out of the fuel wells the floats drop, which opens the needles to allow the fuel to be replenished.  The two fuel wells per throttle body supply the three throttle bores with fuel delivery shared in this fashion: one fuel well supplies the main jets for two cylinders while the other fuel well supplies fuel to the main jet for the third cylinder plus supplying fuel for the accelerator pump circuit…this awareness may be useful in diagnosing carburetor issues.

The three-barrel Weber carburetor uses three different circuits for fuel delivery to the engine; the first two use air flow to create a pressure drop to mix air with fuel while the third circuit delivers raw fuel with each depression of the throttle pedal, these three circuits are: Idle/Progression circuit, Main circuit and Accelerator circuit.  There are no devices to help enrich the mixtures for cold starting; actuating the accelerator pumps is the method relied upon for this situation.

Idle and Progression circuit:  The idle and progression circuit is designed to deliver fuel to the engine for operation from idle to the initiation of the main circuit and a little beyond. When the throttle plates are nearly closed (during slow engine speed operation) they nearly block off the main bore of the throttle body except for a small crescent shaped area where the air must pass through.  Since the demand for air drawn into the engine is greater than what can pass through the crescent the air pressure below the throttle plates is lower than atmospheric air pressure.  In the effort to supply air to the lower pressure side of the throttle plates the velocity of the air increases dramatically as it passes through the crescent shaped clearances.  When air is flowing at a high velocity it results in a low pressure which enables the high velocity.  It is this resulting low pressure that then draws fuel out from little holes (progression ports) in the throttle body wall, which is supplied from the main fuel well.  The fuel that is supplied to the idle and progression circuit is emulsified or mixed with atmospheric air prior to passing through these progression ports to help in the further mixing that occurs in the low-pressure area below the throttle plate.

Main circuit:  As the throttle plates continue to open and the engine speed increases, the pressure difference between the main throttle bore (which is nearly equal to atmospheric air pressure) and the air pressure below the throttle plates decreases, which reduces the amount of fuel flowing out of the progression ports.  At the same time the progression ports slow in their rate of fuel delivery, the increased air flow into the engine is beginning to create another low pressure region within the throttle bore.  This region of low air pressure results from high velocity air flow and occurs at the "waist" or the reduced diameter of the main venturi.  A venturi is a device that causes a fluid to accelerate as a function of its passing through a reduced area in the flow path.  The internal reduced diameter or “waist” in a venturi is the point of maximum airflow velocity and is also the point of minimum pressure.  This reduced pressure is used to draw fuel from the main fuel well, like sucking a beverage through a straw.  As in the idle and progression circuit, the fuel delivered via the main circuit is emulsified or mixed with atmospheric air prior to its introduction into the throttle bore.

Accelerator circuit:  A third fuel delivery system is incorporated which does not rely upon air flow to supply fuel into the throttle bore of the carburetor, this system is referred to as the accelerator circuit.  The accelerator circuit injects raw fuel into the throttle bores above the throttle plates to help bridge mixture deficiencies during rapid throttle actuation.  Rapid throttle opening creates an immediate air flow increase which is greater than the instantaneous fuel delivery capacity from the progression circuit or main circuit; the accelerator circuit comes into play by providing fuel for the engine until the main circuit can supply fuel commensurate with the air flow.

 Detailed Operation

The previous section provided introductory discussions for carburetor operation and of the three fundamental fuel delivery circuits.  The following discussions are more thorough and worthwhile to help understand the three circuits and their interaction with each other.  This aids the driver of the car to understand the differences in fuel delivery methods during different phases of engine RPM vs. throttle position.  Also, the tuner will benefit in the discussions to help better understand what components may be adjusted to help correct running deficiencies due to a fuel delivery profile that is not optimized for a particular engine and the performance expected from it.

Idle and Progression Circuit

The idle and progression circuit provides fuel delivery for engine operational speeds from idle through 3500 RPM (approximately) under partial throttle openings and continues to provide some fuel delivery up through 4500 RPM.  When Wide-Open-Throttle (WOT) is used the fuel delivery is supplied "primarily" from the main circuit; "primarily" as there is a continuance of fuel delivery from the progression circuit but its contribution is minimal. The idle and progression circuit is comprised of an idle jet and its holder, an idle air bleed jet (pressed into the top of the main throttle body, the idle mixture adjusting screw, the idle air adjusting screw and the progression holes drilled into the throttle bore, all of which are supplied with fuel delivered through the main jet and via fuel delivery galleries.  These progression holes are found behind the slotted screw directly above the idle mixture control screw.  By changing the orifice size of the idle jet and that of the idle air bleed jet and by making adjustments of the idle mixture screw and the idle air correction screw the tuner may adjust fuel mixture strength from idle operation through progression and ending with transition with the main circuit.  Typically the idle mixture screw will be 1 ½ to 2 ½ turns open, the idle air adjusting screw will be zero to one turn open and the throttle stop screw will be adjusted to open the throttle lever ¾ to 1 turn after contact.  These are rough settings but if the final settings vary much from these then thought should be directed to selecting new jetting or to diagnosis of why the deviations are required.

When the engine is idling, the throttle plates are nearly closed which creates a strong vacuum in the intake manifold below the throttle plate, the suction from this vacuum draws the fuel from the fuel well and into the throttle bore below the closed throttle plate. The fuel passes up a dedicated fuel gallery located between the emulsion tube well and the fuel gallery running down the outside of the throttle body.  It then passes through the idle jet where atmospheric air from the idle air bleed jet mixes with and emulsifies it before continuing down the external fuel gallery.  The resulting emulsified air/fuel mixture flows out of the metering hole controlled by the idle mixture screw and the progression holes in the inner throttle body wall below the edge of the throttle plate.  The idle mixture screw is a flow controlling needle valve with a tapered tip that mates with a small hole in the throttle bore and once set is secured with pressure from the compression spring wrapped around it.   When the throttle plates are closed during idle operation the first (lowest) progression circuit hole should be blocked by the edge of the throttle plate leaving only the fuel from the idle mixture screw port available for running.  

As the throttle plates are opened up and the engine speed increases there are more holes of the progression circuit exposed to the vacuum below the edge of the throttle plate.  The additional exposed holes supply more fuel to match the increased airflow from the opened throttles.  However, the vacuum below the throttle plates decreases with opening of the throttles until eventually the vacuum is no longer sufficient to continue to draw fuel from the progression circuit.  Eventually all of the progression holes are exposed to the air flow past the opened throttle plates and with larger throttle openings the fuel flow out of these progression holes essentially ends.  Before ending fuel delivery via the progression circuit, the main circuit begins fuel delivery, this simultaneous region of fuel delivery operation is referred to as transition.  Higher engine speeds are only possible by fuel delivery from the main circuit.

Remember that the idle and progression circuit emulsifies the fuel delivered to the engine just as the main circuit does with its air correction jet mixing air with raw fuel via the action of the emulsion tubes.  The idle and progression circuit achieves the same result using a different method; air from the idle air bleed jet is mixed with the raw fuel inside the body of the idle jet.  In addition to this initial emulsification of fuel there is additional emulsification that is far more subtle; progression holes above the closed throttle plate are essentially at atmospheric pressure and those that are below the edge of the throttle plate are exposed to the vacuum of the intake tract.  Therefore, those holes above the throttle plate actually provide additional air to the emulsified fuel in the fuel gallery which leans out the mixture delivered to those holes below the throttle plate.  As the throttle plate is opened and more progression holes are exposed to the vacuum, there are fewer holes exposed to atmospheric air pressure above the throttle plate, this decrease in the number of holes to atmospheric air pressure decreases the air added to the fuel in the fuel gallery.  This is thereby an enriching action and corresponds to the need for a stronger fuel mixture strength with the increased airflow of larger throttle openings.

The idle air adjusting screw provides the ability to match air flow through each barrel of the carburetor at idle speeds.  Besides providing the balancing of air flows these screws perform an often overlooked task in that they allow for setting the throttle plate positions at idle to block fuel delivery from the progression fuel holes.  The air is drawn into the low-pressure region below the throttle plates from the atmospheric pressure above them via a gallery for the purpose of balancing airflow.  The idle air screw has a tapered tip for metering control and once set is secured with a lock nut.

Main Circuit

The main circuit is comprised of a main jet and its holder, a main air correction jet (screwed into the top of the throttle body into the emulsion tube well), emulsion tube, main venturi and the auxiliary venturi.  The main jet is screwed into the tip of the jet holder and the resulting assembly is screwed into the bottom of the float bowl where it is immersed in fuel.  Air flow through the main venturi is generated by the intake stroke of the engine.  The intake air flow creates low air pressure at the waist of the main venturi and by the low air pressure it generates draws fuel through the emulsion tube well which defines main circuit operation.  Fuel flows through the opening in the fuel well where the main jet assembly resides and is drawn into the hollow body of the main jet holder.  From there it flows through the main jet and then vertically into the annular space between the emulsion tube well and the outside diameter of the emulsion tube.  Atmospheric air is also drawn down through the main air correction jet as a result of the suction created within the main venturi.  From the air correction jet it flows down into the inside diameter of the emulsion tube where it escapes through the holes in its body.  This escaping air mixes with the raw fuel in the annulus of the emulsion tube well thereby emulsifying it in preparation for combustion. The emulsified fuel continues upward in the annulus of the emulsion tube well until it reaches the height where it begins to feed into the hollow passageway in the wing of the auxiliary venturi.  The emulsified fuel in the wing is drawn into the throttle bore at the waist of the auxiliary venturi, which is the lowest pressure region within this venturi.  The emulsified fuel becomes atomized by the low pressure and the high speed air flow through this venturi.  The atomized fuel exits the bottom of the auxiliary venturi, coinciding with the waistline of the main venturi, which is the lowest pressure region within the main venturi.  This further atomizes the fuel.  

The result of locating the bottom of the auxiliary venturi at the waistline of the main venturi is an enhanced vacuum signal to help draw fuel from the emulsion tube well. The use of an auxiliary venturi thereby provides an earlier fuel flow than what could be achieved by using the vacuum from the main venturi solely for this purpose.  It follows that larger venturis provide less vacuum to draw fuel from the emulsion tube well than smaller main venturis resulting in a softened throttle response in these situations. 

Fuel delivery is tailored to suit the requirements of the engine by adjusting the sizes of the main and air correction jets and by the selection of emulsion tube.  Main and auxiliary venturi selections also influence main circuit timing and engine power output.

The initiation of flow through the main circuit begins at the same time the effectiveness of the idle and progression circuit begins to decrease.  The fuel delivery during this transition phase of fuel delivery is the summation of fuel flow from both circuits.  If the fuel delivery does not match the engine requirements then there may be a lean or rich situation, which is commonly referred to as a “flat spot” and occurs anywhere from 2500 RPM to 3500 RPM depending on engine characteristics.  The problem may be any combination of the following: the emulsion tube is wrong, the main jet size is wrong, the main air correction jet is wrong or the idle and progression circuit is not providing the right amount of fuel to balance out that delivered by the main circuit components. Adjusting emulsion tubes may fix the issue but adjusting the idle and progression circuit jetting and/or needle settings may work as well.  Also, the main venturi size may be adjusted to affect timing of the effectiveness of the auxiliary venturi suction action.

The primary difficulty in achieving a suitable mixture for the main circuit is selecting the various jets to meter the fuel flow to match the in-rushing airflow.  Although this sounds simple enough it is in fact quite a challenge due to the ever-increasing airflow rate as the engine speeds up.  So, as the air flow rate increases so must the fuel flow rate but since the fuel is a liquid and is denser than air (specific gravity ratio of 557 to 1; fuel to air) the rate of fuel flow will not have a constant relationship with air flow rate.  It is the task of the emulsion tube and the main air correction jet to provide the variable fuel flow needed to match airflow into the engine.  The simple description of this is to consider the situation of sucking a fluid up a tube as in drinking from a straw.  If the straw had a hole in the side you would need to suck harder to get the fluid up the straw and the fluid would be changed by the incorporation of air into the mixture.  The air correction jet and the holes in the emulsion tube provide a similar effect by increasingly exposing more holes of the emulsion tube to air the fuel becomes increasingly harder to suck into the engine and thereby keeps the mixture constant by careful selection of jetting and emulsion tubes.

The emulsion tube wells are located close to each throttle bore and extend down to allow fuel to flow into them via the main jet ports.  The emulsion tubes drop into the tops of these wells and are secured in place with the main air correction jets.  The tubes have a series of holes located along their length and vary in quantity, diameter and relative height.  An additional design feature of the tubes is outside diametric variations, which resemble a collar.  The length and location of this stepped outside collar provides additional tuning subtleties. 

The main jet carrier threads into a bung at the bottom of the fuel bowl and there is no seat for the main jet to seal with the throttle body.  Therefore, the only seal that exists between the main jet and the fuel delivered to the engine is the threads between the main jet holder and the throttle body.  Obviously a quality fit is required to avoid fuel delivery past the threads that would otherwise upset the metered fuel flow through the main jet.

Accelerator Circuit

The accelerator circuit provides the fuel delivery required to eliminate hesitation during acceleration when the throttles are rapidly opened from a partially closed position.  As mentioned in the discussion of progression circuit operation, the fuel delivery is progressive and matched to incremental throttle positions.  A rapid opening of the throttles would upset this balance since the fuel cannot respond quickly enough to maintain proper mixture.  When the throttles are rapidly opened the airflow past the edge of the partially closed throttle plate is replaced with the flow characteristics of main circuit operation; the fuel supply from the idle and progression circuit ceases and main circuit fuel flow is inactive.  Since a rapidly increased throttle opening does not correspond to a matched engine RPM increase there isn’t enough airflow through the main venturis to activate the main circuit due to the lag in time it takes for the main circuit to respond.  The situation of a large throttle opening and little fuel flow generates a lean hesitation if a mechanism to compensate for this transition condition wasn't available.  This is where the accelerator circuit comes into play by injecting a squirt of raw fuel into the airflow via small squirter jets thereby bridging the momentary change of fuel flow through the main venturis. 

Fuel is drawn from the fuel bowl through a check valve in the bottom of the bowl and into the outer portion of the accelerator pump body and is kept ready for demand by upstream check valves (one in the bolt that secures the squirter jet and another being a flapper valve within the inner pump body).  When demanded, the check valve in the fuel bowl closes and fuel is pumped into the inner portion of the pump body, which opens the flapper valve thus allowing the fuel to enter discreet galleries leading to the three squirter jets.  The check valves at the squirter jets open under the force of the pumped fuel and fuel is then injected downward into the annulus between the waist of the main venturi and the outside diameter of the auxiliary venturi. 

Every increment of throttle shaft rotation injects fuel; a sustained rotation injects a continuous supply until the available volume of fuel is depleted.   A linkage mechanism operates the pump and is comprised of a lever arm mounted on the throttle shaft, a pump rod connecting the lever arm to a lever cam mounted to the exterior of the throttle body and a lever arm in the cover of the pump assembly with a roller or plain cam follower which rides on the lever cam.  Adjustments to the accelerator pump system are achieved by multiple components including: squirter jet type, squirter jet nozzle size, cam lever selection and by adjustments made to the accelerator pump rod length via the adjusting nut.  Additional adjustment for strength and duration of the injected fuel may be achieved by selection of the internal spring behind the disc valve in the pump body.

New gaskets don’t allow as much volume of fuel to be drawn into the first chamber of the accelerator pump body as those with some miles on them.  It is therefore important to know that it may be difficult to get a full measure of fuel after a rebuild and that the injection amount will change and need readjustment once the gaskets have achieved a broken-in condition.

Fuel Delivery System Components

The three main fuel delivery circuits are all supplied by fuel in the fuel bowls.  The fuel delivery system maintains the fuel in the bowls at a constant level, which is of paramount importance for uniform fuel delivery to all cylinders and for proper metering of the fuel delivery circuits.  The fuel delivery system is comprised of the following components: needle valve, float and fuel pump system.

Fuel is pumped from the fuel tank and filtered before being delivered to the carburetors at a constant (ideal) pressure of 3.56 psi.  This fuel is allowed to flow into the fuel wells when demand by the engine has decreased the fuel level in the wells thereby allowing the fuel floats to drop and decrease the pressure on the needle valves.  The fuel valves open which allows fuel to flow into the wells to maintain a constant fuel level.  “A constant level” is the important operative phrase here since a constant level controls when the main circuit may be activated.  Remember that fuel in the emulsion tube well is drawn into the carburetor by vacuum; so if the fuel levels are different from one fuel bowl to another then the main circuits will also vary in “timing” of their activation since lower fuel levels require more vacuum to draw the fuel into the engine than higher fuel levels would require.