Post by Scoutpilot on Jan 9, 2015 10:09:38 GMT -5
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 carburetor’s 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.
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.
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.