A Big Thank You to www.HowStuffWorks.com for the following information:
If you have read the page entitled How a Gasoline Engine Works (now newly illustrated with cool 3-D graphics, by the way!), you know that the idea behind an engine is to burn gasoline to create pressure, and then to turn the pressure into motion. A remarkably tiny amount of gasoline is needed during each combustion cycle. Something on the order of 10 milligrams of gasoline per combustion stroke is all it takes!
The goal of a carburetor is to mix just the right amount of gasoline with air so that the engine runs properly. If there is not enough fuel mixed with the air, the engine "runs lean" and either will not run or potentially damages the engine. If there is too much fuel mixed with the air, the engine "runs rich" and either will not run (it floods), runs very smoky, runs poorly (bogs down, stalls easily), or at the very least wastes fuel. The carb is in charge of getting the mixture just right.
On new cars, fuel injection is becoming nearly universal because it provides better fuel efficiency and lower emissions. But nearly all older cars, and all small equipment like lawn mowers and chain saws, use carbs because they are simple and inexpensive.
The carburetor on a chain saw is a good example because it is so straightforward. The carb on a chain saw is simpler than most carbs because it really has only three situations that it has to cover:
- It has to work when you are trying to start the engine cold.
- It has to work when the engine is idling.
- It has to work when the engine is wide open.
No one operating a chain saw is really interested in any gradations between idle and full throttle, so incremental performance between these two extremes is not very important. In a car the many gradations are important, and this is why a car's carb is a lot more complex.
You can see the carb for the chain saw in the following two photos
Carb Photo 1 - this is the side that connects to the engine
Carb Photo 2 - This is the side that receives the outside air through the air filter
Here are the parts of a carb:
- A carburetor is essentially a tube.
- There is an adjustable plate across the tube called the throttle plate that controls how much air can flow through the tube. You can see this circular brass plate in photo 1.
- At some point in the tube there is a narrowing, called the venturi, and in this narrowing a vacuum is created. The venturi is visible in photo 2
- In this narrowing there is a hole, called a jet, that lets the vacuum draw in fuel. You can see the jet on the left side of the venturi in photo 2.
The carb is operating "normally" at full throttle. In this case the throttle plate is parallel to the length of the tube, allowing maximum air to flow through the carb. The air flow creates a nice vacuum in the venturi and this vacuum draws in a metered amount of fuel through the jet. You can see a pair of screws on the right top of the carb in photo 1. One of these screws (labeled "Hi" on the case of the chain saw) controls how much fuel flows into the venturi at full throttle.
When the engine is idling, the throttle plate is nearly closed (the position of the throttle plate in the photos is the idle position). There is not really enough air flowing through the venturi to create a vacuum. However, on the back side of the throttle plate there is a lot of vacuum (because the throttle plate is restricting the airflow). If a tiny hole is drilled into the side of the carb's tube just behind the throttle plate, fuel can be drawn into the tube by the throttle vacuum. This tiny hole is called the idle jet. The other screw of the pair seen in photo 1 is labeled "Lo" and it controls the amount of fuel that flows through the idle jet.
Both the Hi and Lo screws are simply needle valves. By turning them you allow more or less fuel to flow past the needle. When you adjust them you are directly controlling how much fuel flows through the idle jet and the main jet.
When the engine is cold and you try to start it with the pull cord, the engine is running at an extremely low RPM. It is also cold, so it needs a very rich mixture to start. This is where the choke plate comes in. When activated, the choke plate completely covers the venturi. If the throttle is wide open and the venturi is covered, the engine's vacuum draws a lot of fuel through the main jet and the idle jet (since the end of the carb's tube is completely covered, all of the engine's vacuum goes into pulling fuel through the jets). Usually this very rich mixture will allow the engine to fire once or twice, or to run very slowly. If you then open the choke plate the engine will start running normally.