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Teacher
Edition Contents
Skill,
Tools, & Craftsmanship
Cutting
& Drying Fish
Sharpening
Nails,
Pegs, & Lashings
Falling
Trees &
Small-Scale
Logging
Guns
Chainsaw
Clutch & Chain
Ice
Pick
Shelters
Wood
Stoves
Wall
Tents
Steambaths
Insulation
& Vapor Barriers
Gas
Lamps & Gas Stoves
Travel
Piloting
A Boat
Boat
Design
Magnetos
& Spark Plugs
Carburetors
Compression
Outboard
Motor Lower Unit
Outboard
Motor Cooling System
Dogsleds
Snowmachine
Tracks
Snowmachine
Clutch
Snowshoes
Winter
Trails
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Activities
- 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?
The needle valves are made of soft material. There should
be a screen in there somewhere. If dirt gets in the little holes
in the carburetor, it will stop the whole operation.
- Look at the aircleaner on several chainsaws. Can you see how
the air flow might be slowed down by a dirty aircleaner? How does
the owner’s manual say to clean it?
The owners manual often says to use compressed air, but lacking
that we usually use a toothbrush lightly.
- 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.)
3 /4 to 1 turn open on both the 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?
The engine speeds up or slows down. The idle set screw is
like fine tuning the throttle. It determines the lowest speed
the engine can run. Watch the throttle move back and forth as
you adjust the idle set screw.
- 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.
A good mechanic uses his/her ear to tune an engine. It takes
practice, but the sounds of a rich or lean engine are distinct.
- 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?
The springs keep tension on the needle valves so they can’t
vibrate open or closed.
- 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?
It gets too much gas.
- 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?
It feels cold because it is evaporating. In the carburetor
evaporation is constantly taking place. The temperature of the
carburetor is reduced far below that of the outside air.
- 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?
With the carb heat on, the air that is pulled into the carburetor
comes off the heated exhaust manifold. This warm air melts any
ice in the carburetor, but warm air is thinner than cold air.
Therefore, the engine isn’t getting enough oxygen to efficiently
burn the fuel. If the engine ran with carb heat on all the time,
it would be inefficient for lack of air.
- 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.
The block that is split into thin pieces will burn faster.
When fuel is sprayed into the cylinder in a mist, it burns faster
and more completely.
- Ask people in the village about the carburetors that came with
the first snowmachines. Are the ones available now better?
The first carburetors on many snowmachines were Tillotson
carburetors that could operate in any direction. They didn’t
have a float. But they did have lots of tiny holes and parts that
plugged with frost. They were terrible. Most carburetors now are
Mikuni type, with a bowl, float, and simple parts.
Student Response
- A carburetor mixes what and what?
Air and fuel
- What happens if there is too much fuel? Not enough fuel?
Too Much: If there is too much fuel (rich), combustion will
not be complete, power will diminish, and carbon will quickly
build up in the cylinder.
Not Enough: Lose power fade under load, overheat. A lean engine,
running too hot, is self-destructing. Parts warp, wear, and break.
- Why is a carburetor that isn’t getting enough gas particularly
harmful in a two-cycle engine?
A lean engine runs hot and when a two-cycle engine is lean
on gas it is also lean on oil.
- Draw a carburetor and identify the parts.
- What is the purpose of the air cleaner and what happens when
it is dirty?
It filters dirt, dust, and sawdust from the engine. When it
is dirty the air is constricted and more fuel is drawn into the
engine than normal.
- Describe Bernouli’s principle in your own words.
When air (or other fluid) speeds up, the pressure decreases
- What does the throttle do?
It speeds up the air that is passing through the carburetor,
decreasing the pressure
- What do the needle valves do?
It controls the amount of fuel that can get to the fuel jet.
- What does the choke do?
It reduces the area through which the air can flow, therefore
increasing the velocity of the air and decreasing the pressure.
As the air pressure is reduced, the amount of gasoline taken into
the carburetor is increased.
- What does the float do?
It controls the amount of fuel that gets to the bowl of the
carburetor. It shuts off the fuel if there is enough available.
- Why is it important to increase the surface area of fuel?
When the surface area is high, combustion is more complete.
There is greater power and efficiency.
- What is carb icing?
It is ice that has formed in a carburetor. The evaporation
of fuel takes heat. That reduction of heat cools the air coming
into the carburetor. As the air is cooled, the moisture in it
is condensed and cooled to freezing.
Math
- 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 = 127
$8.31
- 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?
1.8 psi, 1.3 psi
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