|| Engines need fuel, but also need proper amounts of oxygen to burn
the fuel. Carburetors mix air and fuel in the proper amounts to ensure
efficient combustion in the engine.
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.
A 2, 14,
B 1, 3
D 1, 3
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 doesnt 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
are seven important parts of a carburetor.
- Air cleaner
- Carburetor throat and jets
- Throttle butterfly
- Needle valves
- Float or other regulating system
- Throttle cable
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 isnt 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.
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.
Bernoullis 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).
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.
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
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.
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
- 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.
- 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 couldnt be turned upside down and continue
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
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.
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 rpms. The oil injection varies the amount of oil at
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
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
This is why airplane engines have a carb heat control
to inject warm air, melting any ice formed in the carburetor.
- 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. Dont 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?
- 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 owners manual say to clean it?
- Look in the owners 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.)
- 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?
- 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.
- 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
isnt too lean. Why do you think there are springs on the
needle valves if they arent moving parts?
- 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?
- While the chainsaw is running, pull the throttle. Look in the
carburetor. Can you see the throttle butterfly moving?
- Put a little gasoline in your hand, and blow on it. Does it
feel hot or cold? Why? Can you understand carb icing now?
- 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 dont
run with carb heat all the time?
- 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.
- Ask people in the village about the carburetors that came with
the first snowmachines. Are the ones available now better?
- A carburetor mixes what and what?
- What happens if there is too much fuel? Not enough fuel?
- Why is a carburetor that isnt getting enough gas particularly
harmful in a two-cycle engine?
- Draw a carburetor and identify the parts.
- What is the purpose of the air cleaner and what happens when
it is dirty?
- Describe Bernoullis principle in your own words.
- What does the throttle do?
- What do the needle valves do?
- What does the choke do?
- What does the float do?
- Why is it important to increase the surface area of fuel?
- What is carb icing?
- 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
- 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?