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FLOAT SYSTEM (Figure 10)


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 needle seat. This float action, controlling needle valve movement, permits fuel to enter or be stopped according to float height.


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 enter 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 allowing the weight of the needle valve to move it away from the orifice hole in the needle seat. The force of the incoming fuel will also assist in moving the needle valve downward. Some carburetors will employ the use of a "pull down clip," which connects the needle valve to the movable float arm, resulting in an immediate positive opening of the needle valve as the float is lowered. This action prevents any possi­bility of a momentary lowering of the fuel level due to a delayed opening of the needle valve.

Fuel will then enter the float bowl through the open needle 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 the needle seat. When a predetermined level has been reached (determined by the float level adjustment), the tip of the needle valve will contact the needle seat closing the orifice hole preventing any more fuel from entering the carburetor bowl. As engine demands consume more fuel, the float will again be lowered allowing the needle valve to move off its seat, repeating the cycle.

Float Drop:

A float drop tang located at the rear of the float arm 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 demands.

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 internal and the fixed external vent. The external vent, located in the 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 bowl chamber due to high under-hood temperatures. If these vapor pressures are not disposed of, then the pressure inside the bowl pushing down on top of the level of the fuel will be greater 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 and on into the intake manifold.

Balance Tube:

All carburetors will employ an internal vent which is in a sense a balance tube balancing the pressure on top of the fuel with the pressure of the air 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 of the fuel in the float bowl with the pressure of the air as it is drawn into the carburetor. This action will compensate for a dirty or restricted air cleaner. Without this connecting or internal vent tube it is conceivable that, at sea level, there could be a pressure of 14.7 lbs. per square inch pushing down on top of the fuel in the float bowl, but only a pressure 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 calibration of the carburetor.

Idle Vent Valve:

Most carburetors today will use a movable atmospheric idle vent valve in place of the fixed external vent system. This movable valve will be held 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 throttle linkage is progressively opened, the actuating lever from the linkage will no longer contact the idle vent arm 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 during all periods of operation except at idle, otherwise excessive richness can be caused by the higher atmospheric pressure acting upon the fuel in the float bowl.

Built-In Fuel Filter:

From the opening and closing action of the needle and seat asembly, we can see why it is so important that the fuel be filtered and all 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, the 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 needle valve to open just far enough to replace the fuel at the same rate at which it flows from the fuel bowl into the engine. Under idle or low-speed operation when very little fuel is needed, the needle valve will assume a slightly open position allowing only a small amount of fuel to enter. If the incoming fuel is contaminated with dirt particles which could collect at the needle tip preventing the tip from assuming a close enough position into the seat, it is possible for more fuel to enter than is required by the engine. As the level of the fuel rises in the float bowl, the dirt 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 top of the high-speed jet and on into the engine. We would now 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 engine would actually stall at idle and low speeds. Since the fuel from the fuel pump could still be under pressure it will continue to enter the carburetor bowl, dumping more fuel 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 close. Without this resiliency and dirt-absorbing action a flooding condition would result. As the fuel is consumed from the fuel bowl, the float drops, and the needle 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 (Figure 11)


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.


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 flowing 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 beneath 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.

OFF - IDLE SYSTEM (Figure 12)


As engine speed slowly increases due to the further opening of the throttle valves permitting more air to enter, an additional quantity of fuel is needed to combine with the extra air. This is accomplished by the secondary idle discharge holes. Fuel flow for off-idle operation is identical to the idle system with exception of fuel flowing from the secondary idle discharge holes as well as the regular idle holes. As the throttle valves move past the secondary idle discharge holes, they become progressively exposed to the area of low pressure (or high vacuum) and begin discharging fuel where previously they acted as air bleed holes.

Because air velocity through the venturi is not great enough to cause fuel to flow from the main metering system, the regular idle and secondary idle discharge holes continue to supply sufficient fuel for engine requirements. Some carburetors use slotted secondary discharge ports in place of conventional discharge holes, but either type will give the correct air/fuel mixture ratios. As a refresher we will review the flow of fuel once again.


The fuel will flow from the fuel bowl through the metering jet into the main well area. From there it will flow up through the idle tube to the top of the idle passage in the venturi cluster, where it is bled with air coming in from the first idle air bleed hole. This air, as was previously stated, tends to break up or help in the atomiza­tion of the fuel for easier burning. The fuel and air mixture will now flow across the cross channel where it is bled a second time by air enter­ing the second idle air bleed hole. The fuel is further mixed with air at this point and atomization is further accomplished.

The fuel and air mixture will now travel down the idle passage to a point where it meets the calibrated idle restriction. The low pres­sure created at the calibrated idle restriction causes the fuel to flow faster through the smaller opening which in a sense aids further in the atomization of the fuel. As the air/fuel mixture travels down the idle passage, it now reaches the location of the secondary idle discharge holes where it is drawn into the bore of the carburetor supplementing the fuel being discharged through the idle discharge hole.

As the fuel mixture is ejected into the rapidly moving air stream, the violent turbulent action further results in the required change of fuel from a liquid to a fine mist state. When this fine mist enters the intake manifold and is subjected to the heat of the engine, it is then immediately changed into a vapor form. We now have the final step in the transformation of fuel from a heavy liquid state to a light vapor form. This vapor will now readily burn when compressed in the engine cylinders and ignited by the spark.



The purpose of the idle air by-pass system is to provide air for the engine at idle speeds through a separate passage other than past the slightly open throttle valves as previously described in the conventional idle system. It has been felt by carburetor engineers that this method of regulating idle speed with fully closed throttle valves will retard the formation of carbon and gum deposits which normally tend to collect around this area. If permitted to form, these carbon and gum deposits will decrease the area at this point resulting in rough engine idle.

It will be noted that the fuel flow through the idle circuit is identical to that shown in the basic idle system with the exception that the throttle valves are now in a fully closed position. Although there is a hole drilled through each throttle valve to maintain a constant idle air flow for part of the idle air requirements, this hole or fixed air bleed is not large enough to permit enough air to enter to sustain engine idle. Therefore, other means of allowing more air to enter the engine must be provided. This is accomplished through the idle air by-pass passage.


Idle Air Adjustment Screw:

The amount of air that can be drawn into the engine through this passage is controlled by the idle air adjusting screw which is located in the float bowl casting at the rear of the carburetor. By rotating this screw inward or clockwise, we decrease the opening thereby decreasing the amount of air that can enter. Consequently, by restricting the amount of air, the engine idle will decrease. By rotating the idle air adjusting screw outward or counterclockwise, we allow a larger volume of air to be drawn into the engine and, therefore, engine speed will increase.

Idle Mixture Screws:

By adjusting idle speed with the idle air adjusting screw, the mechanic must be aware of the following condition. By rotating the idle air adjusting screw outward to increase engine speed, by allowing more air to enter the manifold, we must compensate for this additional air by re-adjusting the idle mixture screws to allow more fuel to be drawn inside the engine. If engine speed is too great and the idle air adjusting screw is rotated inward to decrease the idle speed, then the idle mixture screws must also be adjusted inward to compensate for this reduced amount of air. If this procedure is not followed, then the idle mixture will be too rich or too lean, depending upon the amount of air entering the engine. This system is used on some Rochester and Carter carburetors.

Secondary Idle Holes:

Because the throttle valves are completely closed and engine manifold vacuum is being confined beneath the closed throttle valves, there is no fuel flow from the secondary idle discharge holes.

The secondary idle discharge holes will continue to act as air bleed holes as long as the throttle valves remain closed. As the throttle valves are opened to increase engine speed, the vacuum in the intake manifold is permitted to reach the area of the secondary idle discharge holes. They will then being discharging fuel to satisfy increasing engine demands.


next: Main and Power Circuits


Table of Contents


Basic Carburetor Theory and Terms

Vacuum and Atmospheric Pressure

Principle of the Four Stroke Engine

Venturi Principle - Principles of Carburetor Operation

Theory of Operation

Main and Power Circuits

Accelerator Pump System

Choke Circuit