Only in outboard motor operation does the speed remain fairly constant. In chainsaws, four-wheel ATVs, and snowmachines, the engine speed is constantly changing.

Mixing fuel and air properly at all rpms is a challenge.

Improper Mixture

Too Much Fuel

If there is too much fuel (too rich), combustion will not be complete, power will diminish, and carbon will quickly build up in the cylinder.

Not Enough Fuel

If an engine doesn’t get enough fuel (too lean), it will lose power, fade under load, and overheat. Proper mixture at all speeds is important. A lean engine, running too hot, is self destructing as parts warp, wear, and break.

We must also remember that a two-cycle engine mixes the oil and fuel. An engine that is lean on fuel is also lean on oil. If it is lean on oil, friction does its irreversible damage.

Parts of a Carburetor

There are seven important parts of a carburetor.

• Air cleaner
• Choke
• Carburetor throat and jets
• Throttle butterfly
• Needle valves
• Float or other regulating system
• Throttle cable

Air Cleaner

The air cleaner is an important part of the carburetor system, especially in chainsaws where there is so much sawdust in the air. If sawdust or dirt are drawn into the carburetor, the carburetor plugs up and sawdust quickly wears and destroys the engine.

If the air cleaner is covered with dirt, the air supply is reduced and more fuel is drawn into the cylinder. The engine runs far too rich. An outboard isn’t operated in dusty conditions. Four-wheel ATVs and chainsaws need frequent attention. The air cleaner on a snowmachine can be covered with snow or frost.

Throat of the Carburetor

The throat of the carburetor is nothing more than a narrowed tube. When air passes through the narrow part, the air must speed up.

Bernoulli’s principle says that as a liquid or gas speed up, the pressure is reduced. Because the velocity of the air in the carburetor throat is increased, the pressure is reduced.

As the fast flowing air passes quickly over the high and low speed jets, fuel is pushed through the jets into the low pressure air stream from the bowl below. By the time the fuel is in the cylinder, it has been thoroughly mixed with the air (oxygen).

Throttle Butterfly

As the throttle cable is pulled, the butterfly opens and closes, controlling the airflow. The amount of air and speed of the air flowing over the jets is changed.

Needle valves

As an engine needs more fuel at higher speeds, there are actually two jets, one for low speed and one for high speed. The low speed jet feeds fuel into the air stream at low speeds. At higher speeds, they both do.

There is a screw that adjusts the amount of gas available to the jet. It is called the “needle valve” because the end of it is thin like a needle. Small adjustments of the screw allows precise amounts of fuel to pass the needle valve and go to the jet.

Years ago, both the high and low speed needle valves were adjustable. Now, except on chainsaws, only the low speed needle valve can be adjusted.

When an engine runs lean, the first thing people do is tinker with the needle valves. The main cause of fuel starvation is dirty fuel in the carburetor or a clogged fuel filter. Once an engine is tuned, it seldom needs needle valve adjustment except for extreme temperature differences. Most engines with two needle valves can be roughly adjusted by gently closing both needle valves, and opening 3/4 to 1 complete turn. The low speed valve is adjusted first, then the high speed.

Choke

A cold engine needs more fuel than a hot engine. The remedy for this is the choke. The choke reduces the area the airflow passes through. As with the throttle, the velocity of the air increases, and more fuel is pushed into the throat of the carburetor. When the engine is running and warm, the choke is no longer needed.

Float or Other Regulating System

Although carburetors are different in some aspects, the principles they operate by are the same. There are basically two kinds of shut-off systems:

1. Those with a float shut off. They operate in an upright position only. Snowmachines, four-wheel ATVs, outboards use carburetors with a float that controls the amount of gasoline available to the carburetor. When the bowl is full of gas, the float rises and shuts off the fuel intake to the carburetor. When the amount of fuel in the bowl drops, the float also drops, allowing more fuel to come into the carburetor.
2. Those that can be operated in any direction (omnidirectional). This kind is found in chainsaws, although many of the early snowmachines had them. Air pressure and crankcase pressure open and close small valves and chambers that allow the saw to get the proper amount of fuel at any throttle setting in any position. If a chainsaw had a float, it couldn’t be turned upside down and continue running.

Throttle cable

The throttle cable is a stiff wire that slides within a covering. This attaches the throttle to the carburetor, so the operator is constantly in control of the speed of the engine.

Surface Area of Fuel

It is important that there is great surface area for the fuel to burn. Burning can only take place on the surface of fuel.

If you split a dry block of wood into many small pieces, it will burn much faster than if it is burned in one whole piece. Liquid fuel, like gasoline, will burn quicker if it has more surface area. If a stream of gasoline is injected into the cylinder, it burns much slower than the same amount of gasoline that has been sprayed into a mist.

Oil Injection

Like snowmachines, newer outboard motors have oil injectors that mix the fuel and oil. The ideal oil/gas mix is different at high and low rpm’s. The oil injection varies the amount of oil at different speeds.

Icing

Heat is required to turn a liquid into a vapor. Assume the carburetor, fuel, and air are at fifty degrees. The fuel is vaporized in the carburetor. It takes heat to evaporate a liquid to a vapor. The heat comes from the carburetor walls. As this process continues, the carburetor actually gets ten to fifteen degrees colder than the outside air. The carburetor cools the air passing through the carburetor throat.

As warm air holds more moisture than cooler air, the air, now cooled in the carburetor, releases its moisture. It can actually form ice in a carburetor when the outside temperature is forty to sixty degrees!

This is why airplane engines have a “carb heat” control to inject warm air, melting any ice formed in the carburetor.

Activities

1. Find an old carburetor from any machine that utilizes a float. Identify the parts. Identify how the float controls the amount of gas in the bowl. Is there an artificial rubber seal to shut off the flow of fuel? Take the needle valves out. Draw the shape of the tip. Don’t touch the tip with a file, but touch the side of the needle valve. Is it hard or soft? Can you find a screen in the fuel line within the carburetor? What do you think would happen if this became plugged?
2. Look at the air cleaner on several chainsaws. Can you see how the air flow might be slowed down by a dirty air cleaner? How does the owner’s manual say to clean it?
3. Look in the owner’s manual of a chainsaw. What is the standard setting for the needle valves? (If no chainsaw is available, try to find another engine that has a carb with a high and low speed needle valves.)
4. Take the bar and chain off a chainsaw. Replace the clutch cover (for safety reasons). Remove the cover from the carburetor. Start the engine. Find the idle set screw. Adjust it when the engine is idling. What happens?
5. Set the high-speed needle valve too rich and then speed the engine up. Can you hear the sound when it is getting too much gas? Now shut the high-speed needle valve down. Speed the engine again. Can you hear the weak sound it makes? These two sounds will help you tune engines in the future. Remember them.
6. We usually set the needle valve halfway between the points where we can hear the lean weak sounds and the rich sounds. Then we open the needle valve 1/4 turn. This insures that the engine isn’t too lean. Why do you think there are springs on the needle valves if they aren’t moving parts?

7. While the chainsaw is running without a bar and chain, remove the air cleaner. Pull the choke lever. Can you see the choke butterfly? Why do you think choking a warm engine kills it?
8. While the chainsaw is running, pull the throttle. Look in the carburetor. Can you see the throttle butterfly moving?
9. Put a little gasoline in your hand, and blow on it. Does it feel hot or cold? Why? Can you understand carb icing now?
10. The next time you are in a small plane, ask the pilot to show you the carb heat knob. Ask him why the engine loses a little power when it is applied. Does this explain why pilots don’t run with carb heat all the time?
11. Cut two identical blocks of wood. Split one into four parts, and the other into kindling. Make two separate campfires and burn them at the same time. Which one burns faster? Explain to someone else why fuel is sprayed into the carburetor in a fine mist.
12. Ask people in the village about the carburetors that came with the first snowmachines. Are the ones available now better?

Student Response

1. A carburetor mixes what and what?
2. What happens if there is too much fuel? Not enough fuel?
3. Why is a carburetor that isn’t getting enough gas particularly harmful in a two-cycle engine?
4. Draw a carburetor and identify the parts.
5. What is the purpose of the air cleaner and what happens when it is dirty?
6. Describe Bernoulli’s principle in your own words.
7. What does the throttle do?
8. What do the needle valves do?
9. What does the choke do?
10. What does the float do?
11. Why is it important to increase the surface area of fuel?
12. What is carb icing?

Math

1. A carburetor is set too rich. It uses 7% more gas than it should. The operator spends $127 on gas in one month. How much could he save by tuning his carburetor? 1.07x z 127
2. The pressure in an airplane carburetor throat is 12.9 psi. Atmospheric pressure is 14.7 psi. What is the difference in pressure? The plane climbs; the atmospheric pressure is now 14.2 psi. What is the pressure difference now?