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Carter WO Ė 101

The Carter WO comes in two models, 596S on CJ2As before SN212110 and 636SA.on later CJ2As and all CJ3As. There are some differences but there are insignificant to this article.

This carburetor is very simple as far as carburetors go. There are only five circuits. However, as with all carburetors, it is a precision device and needs to be handled as such. To allow for an easier understanding of the carburetor as a whole, This article will discuss the circuits individually. The five circuits are: float circuit, low-speed circuit, and high-speed circuit, accelerator pump circuit and finally the choke circuit.

To enhance our understanding of carburetor operation, we must understand that the fuel in the fuel-bowl is at atmospheric pressure. This pressure is about 14.7 pounds per square inch absolute (PSIA). The engine and venturi of the carburetor creates a low pressure. The atmospheric pressure actually pushes the fuel up passageways into the throat of the carburetor where it is mixes with air to create a combustible mixture.

The proportions of this mixture are critical and the carburetorís job is to keep them optimum. To achieve these optimum proportions the orifices or jets in the carburetor are calibrated by the factory. The orifices must be kept at their designed size for the carburetor to perform its function optimally.

Float Circuit:

The float circuit controls the fuel level in the carburetor bowl. If the level is too high excessive fuel will be provided and the mixture will be too rich. Conversely, too low of a level will cause the mixture  to be too lean. Both of these will lead to poor performance and can be taken to the extreme. An extremely high  fuel level can cause raw fuel to spill into the venturi at idle causing flooding. An extremely low fuel level will result in the negative pressure at the end of the nozzle being insufficient to pull enough fuel from the bowl to allow the engine to do anything but Idle.

Fuel enters the fuel bowl through a valve called a needle and seat. The needle is pressed against the seat by the float to shut off fuel. When the fuel level drops to a level where the needle moves away from the seat, fuel enters the bowl until the needle moves to the seat, thus shutting off fuel. The needle assembly is a three-piece device consisting of a pin, spring and needle. The float actually presses the pin and the spring dampens the bumps found when driving off-road.

Fuel pressure plays an important part in the fuel level also. Too much pressure will cause a high fuel level and too little pressure will provide too low of a fuel level. A low fuel pressure could also be insufficient to provide the fuel required by the engine. Stalling could occur. The size of the orifice in the needle and seat assembly also effects the fuel level. Too large of an orifice will act like too much pressure and too small of an orifice will act like too little pressure. This is covered in detail here.

Low-Speed Circuit:

The low speed circuit is sometimes called the idle circuit. This circuit provides fuel to the engine from idle until the high-speed circuit kicks in at about 20 mph. The low speed circuit actually supplies some fuel during the operation of the high-speed circuit. The significance of this is very small after the high-speed circuit begins to operate.

Intake manifold vacuum causes fuel from the fuel bowl to flow through the idle well jet and up the low-speed tube. Air enters through a throttle-plate bypass port and mixes with the fuel creating a fuel-rich air-fuel mixture. This mixture flows through an orifice known as the economizer. It continues past the air bleed and down to the idle port and idle adjustment needle opening. The idle-mixture adjustment screw provides the adjustment necessary at idle.

The idle port is a slot that is partially covered by the throttle plate. Air from the top of this slot mixes with the air-fuel mixture and finally achieves the correct proportions for combustion. As the throttle plate is opened, more of the slot is exposed. This allows more of the fuel-rich mixture to flow into the engine and the increased air, allowed by the open throttle-plate, leans the mixture to the correct proportion. This increase continues until the high-speed circuit can take over. Actually, the idle circuit continues to provide a small amount of fuel through the operation of the carburetor.

High-Speed Circuit:

The high-speed circuit begins to operate at about 20 mph. At this point, the velocity of air passing through the venturi of the carburetor reduces the pressure at the main nozzle and fuel begins to flow through the metering rod orifice. It continues past the metering rod and out the main nozzle where it mixes with the air flowing through the venturi. The correct amount of fuel required by the amount of air flowing through the throat is controlled by the taper of the metering rod.

The metering rod is inside the metering rod orifice and tapered so as the throttle is opened wider more fuel passes through. At wide-open throttle (WOT) only the small tip of the metering rod is inside the metering rod orifice.

Accelerator Pump Circuit:

The accelerator pump circuit performs two functions. These include additional fuel during starting and additional fuel when accelerating quickly.

During the engine starting operation, the intake vacuum is quite low. Therefore, neither the low-speed nor the high-speed circuits can supply an air-fuel mixture. When depressed, before starting the engine, the accelerator pump supplies a small amount of fuel to the intake manifold where the vapors are pulled into the cylinders during the cranking operation. This, along with the choke circuit (described later), assists in engine starting.

When the throttle is opened the high-speed circuit reaction time doesnít react as quickly as required. The accelerator pump provides increased fuel to the throat of the carburetor during these critical times.

Operation of the accelerator pump is a simple pump circuit consisting of a spring-return cylinder, two check valves, a discharge nozzle and spring to prolong the flow.

When the throttle is opened the cylinder rod is forced down forcing fuel in-between the check valves. The intake check valve closes and the exhaust check valve opens allowing fuel to flow up to the discharge nozzle in the carburetor throat. When the throttle is opened more quickly than the circuit can discharge fuel, the spring acts to prolong the fuel flow. This precisely what the high-speed circuit requires due to itís relative slow reaction time.

When the throttle begins to close the cylinder reverses itís movement, closing the exhaust check valve and opening the intake check valve. Fuel then flows from the bottom of the bowl through a filter screen to recharge cylinder for the next operation.

Choke Circuit:

The choke circuit is used in starting and warming up a cold engine by providing a fuel rich mixture to the engine. The components of the choke circuit include, the choke plate mounted on a shaft and a lever at one end. A spring attached to the lever is actuated by the choke linkage controlled by the choke cable attached to the knob in the driverís compartment. A connector link is attached to the choke linkage and throttle linkage.

During starting, the engine needs a fuel-rich mixture. When the choke is closed, all vacuum produced by the engine is trapped in the carburetor throat. This enables both the low-speed and high-speed circuits to provide fuel, thus in-richening the mixture. The connector link between the choke linkage and the throttle link opens the throttle slightly to enhance the vacuum above the throttle plate.

Upon starting, the cold engine continues to require a fuel-rich mixture. The choke continues to enhance the vacuum in the carburetor throat where by a fuel-rich mixture continues. A bypass on the choke plate to restricts the vacuum that can be applied.


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Copyright Richard N. Meagley Sr.
Last revised: May 25, 2007.