I.
Course: Ag Mech
Unit: Small Engine Maintenance and Repair
Unit APT6: Internal combustion engines
II.
Situation: Junior or senior levels Ag Mech. Students have been through fundamentals
of small engines and have a concept of the principles of operation and can identify the
parts of a small engine.
III.
Teacher Objectives: After this lesson student will be able to:
1) Identify tools needed for engine maintenance and repair orally with 100%
accuracy.
2) Demonstrate the basic maintenance procedures required for small engines in the
lab with 95% accuracy.
3) Explain the procedures for repairing the systems of a small engine on a written
quiz with 90% accuracy.
4) Establish troubleshooting techniques using sequential procedure in the lab with
95% accuracy.
IV.
Key Terms:
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Air Filter- Separates debris from air taken in during operation
Oil Foam Filter- utilizes oil to catch debris
Dry Element Filter- Various shapes and sizes, constructed of many different
materials
Dual Element Filter- May use a pre-cleaner
Octane Rating- refers to the ability of fuel to prevent “pinging” or pre-ignition of
the fuel in the combustion chamber.
Air-Fuel mixture- the mixing of fuel and air controlled by the carburetor
Overhaul- the removal and repair of a component or components
Service Classification- rating of engine oil based on performance and viscosity
Viscosity- thickness or thinness of engine oil
SAE- Society of Automotive Engineers
Micrometer- measuring tool used to check components for wear
Telescoping Gauge- t-shaped tool used to measure cylinder wear
Plastigage- threadlike material used to measure connecting rod bearing clearance
Torqued- tightening of a fastener to a specific degree
Teaching Procedure:
A. Interest Approach: I will place 2 or 3 small engine repair specialty tools (probably
a plug gaper, valve spring compressor, and a compression tester) on the front table
and ask the students if they recognize any of the tools in front of them.
1. Have you ever seen any of these before?
2. What are they?
3. Are they the same?
4. Can you guess what the function of these tools might provide?
Transition to Reasons to learn: Now that we have established what these tools are,
what might they be used for and why is their use important?
B. Reasons to Learn
1) Why is it important to be able to identify tools needed for engine maintenance and
repair?
2) Why is it important to be able to demonstrate the basic maintenance procedures
required for small engines?
3) Why is it important to be able to explain the procedures for repairing the systems
of a small engine?
4) Why is it important to be able to establish troubleshooting techniques using
sequential procedure?
Transition to Questions to answer: Now that we have established a few reasons
to learn about the fundamentals of small engines what questions might we ask
about usage?
C. Questions to answer
1) What do we need to know in order to be able to identify tools needed for engine
maintenance and repair?
2) What do we need to know in order to be able to demonstrate the basic
maintenance procedures required for small engines?
3) What do we need to know to be able to explain the procedures for repairing the
systems of a small engine?
4) What do we need to know to be able to establish troubleshooting techniques using
sequential procedure?
Now that we have some questions about small engines maintenance and repair
lets answer them.
D. Answers to questions:
1) What do we need to know in order to be able to identify tools needed for engine
maintenance and repair?
Basic Hand Tools:
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Standard wrenches sizes ¼ through 1 in.
Metric wrenches sizes 6 through 24
Needle nose pliers
Locking jaw pliers
Standard and metric hex wrenches
Phillips head screw drivers
Standard screw drivers
Ball peen hammer
Chisels
Punches
Specialty Tools:
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Flywheel wrench or Strap wrench
Flywheel puller
Three jaw puller
Torque wrench
Micrometer
Telescoping gauge
Compression gauge
Spark tester
Ring expander
Ring compressor
Valve grinding tool
Cylinder hone
Spark plug gaper
Spring compressor
Pick set
Magnetic pickup tool
2) What do we need to know in order to be able to demonstrate the basic
maintenance procedures required for small engines?
Read and follow all service publication instructions!
Service manuals are a student’s best friend. Read! Read! Read!
When in doubt read again.
Shop safety and small engines
Like any other lab work danger is always a possibility. Small engines have
multiple moving parts which can be dangerous. Small engines run on gasoline
which is extremely flammable so we must use caution when working on small
engines.
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Safety glasses must be worn at all times.
Keep hands and feet away from all moving parts during operation.
Keep all ignition sources away from the work area.
Stop engine and allow it to cool before refueling.
Handle gasoline outdoors if possible.
Disconnect the spark plug lead when making adjustment to prevent
accidental start-up.
Never disengage any of the safety equipment or devices.
Keep all shields in place.
Do not operate engines above the specified speed.
Never overload engines or force equipment beyond it the designed
capacity.
Only start and run engines where ventilation is adequate, preferably
outdoors.
Never use gasoline as a solvent to clean parts.
Never pour gasoline or oil on the ground or down a drain, dispose of it in
the correct container to be recycled.
We must also be concerned with the basic maintenance items:
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Air filters can become clogged over time and should be changed (Never!
Blow out an air filter with compressed air!) This can damage the filter
and cause immediate engine damage.
Fuel filters can also become clogged over time and should be changed
(Never! Blow out a fuel filter with compressed air!) This can damage
the filter and cause immediate engine damage.
Tanks & valves can also become clogged over time however these items
can be clean with solvent and blown out with low volume compressed air
Carburetors can be adjusted although it is rarely necessary. Commonly
small engines are factory preset and will not acquire adjustment. However,
in the case that adjustment is necessary there are usually 3 adjustment
points on a carburetor.
 Main mixture adjustment screw- this screw adjusts the fuel to air
mixture ratio by allowing more or less liquid fuel into the carburetor to
mix with air to be combusted.
 Idle mixture adjustment screw- this screw adjusts the idle air to fuel
mixture.
 Idle Speed adjustment screw- this screw adjusts the idle speed by
changing the position of the throttle valve.
 Lubrication is very important it is necessary to deduce whether or not the
engine has a pressurized lube system or not. This is a simple deduction.
1. Pressurized systems- utilize the use of an oil filter
2. Non-pressurized- does not utilize an oil filter.
 This is important because different engines require different oils of
different viscosities.
 Viscosity- is the thickness of the oil
 Many non-pressurized engines call for non-detergent, single viscosity oils
such as ND30
 Many pressurized engines call for multi-viscosity oils such as 10w30
 Multi-viscosity- is the thickness of the oil at different operating
temperatures
 Dip sticks must also be located and checked regularly according to service
manual.
 Cooling system- usually can be blown out with compressed air in air
cooled and liquid cooled applications. May require coolant checks for
adequate level and ratio in liquid cooled applications.
 Electrical system- somewhat maintenance free with the exception of the
spark plug which requires replacement at implied intervals.
3) What do we need to know to be able to explain the procedures for repairing the
systems of a small engine?
KIS- Keep It Simple, Always check the easy things first. If an engine suddenly
stops running, chances are the solution is an easy fix.
As we learned in the previous chapter the principles of operation must happen
sequentially for an engine to run. If one or more of these steps are incomplete the
engine will fail to run.
Every engine repair is different just as every engine is different. It may sound
funny but engines are a lot like people. You may have heard someone say it’s just
cold natured. They are referring to the engines need to warm up longer before the
choke is taken off.
Like people engines require care to stay in good shape. The better you take care of
an engine the more longevity you can expect from it.
We must know how to use specialty tools like:
 Micrometer- measuring tool used to check components for wear
 Telescoping Gauge- t-shaped tool used to measure cylinder wear
 Plastigage- threadlike material used to measure connecting rod bearing
clearance
 Torque wrench- used to tighten fasteners to specific tolerances
 Ring-compressor- used to compress piston rings for piston installation
 Flywheel wrench or strap wrench- used to hold flywheel to remove nut
 Flywheel puller- used to pull stuck flywheels
 Spark tester- used to safely test spark
 Spring compressor- used to compress valve springs
 Feeler gauge- used to check clearances
 Valve grinder- used to grind and lap valves
We go about repairing different engine systems in different ways. We must first
learn to trouble shoot and diagnose before we could have any hopes of doing a
repair. If we don’t know what’s wrong how could we ever of fix it?
4) What do we need to know to be able to establish troubleshooting techniques using
sequential procedure?
Troubleshooting and diagnosing engine faults is the most integral portion of
this lesson. We can break troubleshooting down to a few broad points and
then further sharpen those points. Here are the steps in order
1. Check for spark. If the engine cranks but will not start it may not be firing.
Use a spark tester to identify spark. Lack of spark may be a faulty plug,
armature coil, breaker points, or ground switch
2. Check for fuel. Make sure there is plenty of fuel in the tank and that the fuel is
fresh. Remove the fuel line from the carburetor to be sure that the fuel flows
freely from the tank, through the line and into the carburetor. If it doesn’t you
must find the blockage and remove it.
3. If you have spark and fuel but the engine still fails to start there could be other
simple problems. After cranking several times remove the spark plug and
examine it. If it is dry then you have a fuel delivery issue inside the
carburetor. The carburetor should be overhauled or replaced if necessary. If
the plug is extremely wet the carburetor is flooding the engine and fuel is not
being atomized effectively enough to combust. This too requires carburetor
disassembly/overhaul or replacement. Many times on small engines a
replacement carburetor can be purchased at or below the cost of an overhaul
kit plus labor so it makes sense to replace entire unit.
4. If you have spark and fuel but the engine fails to start or sounds like it is
backfiring or is hard to crank, the flywheel key could be to blame. A flywheel
key is designed to take the brunt of a blow if the engine is stopped abruptly.
This design saves more expensive components from being damaged. However
the flywheel key does affect the fundamental operating principals of a small
engine. The flywheel key holds the flywheel in direct correlation to engine
component timing. If the key is sheared the timing will be affected because it
adjusts the position of the permanent magnet found in the flywheel. This
adjustment causes the spark plug to fire at the wrong time hence the
backfiring or no start condition. This is very common and can be easily
remedied by the replacement of the flywheel key.
5. When all of the above methods have been exerted and found to be ineffective,
you should run a cylinder compression test. Compression test results will act
as a guide and let you know which route to take. If the compression tests well
then the first four steps may need to be repeated because you may have missed
something simple. If the compression tests badly then the repair may have just
gotten more costly. Engines lose compression for three reasons, wear and tear,
blown head gasket, or off timing. Wear in the cylinder, piston rings, and valve
guides could be the culprit of low compression. Another common problem is a
blown head gasket, this is when the gasket between the cylinder block and the
head weakens and compression escapes at the weak point. If the timing of an
engine’s valve train can also affect compression. If the timing is off, the
valves may be open during the compression stroke allowing an escape of all
compressed gases. This problem is usually caused by internal component
damage and may result in a very costly repair. In no-tolerance engines if
timing is off the piston could come in contact with the valves and cause
serious component damage.
E. Summary:
1) What do we need to know in order to be able to identify tools needed for engine
maintenance and repair?
Basic hand tools and specialty tools
2) What do we need to know in order to be able to demonstrate the basic
maintenance procedures required for small engines?
Read and follow all service publication instructions!
Service manuals are a student’s best friend. Read! Read! Read!
When in doubt read again. Shop safety and small engines, we must also be
concerned with the basic maintenance items: filters, lubes, dipsticks, etc.
3) What do we need to know to be able to explain the procedures for repairing the
systems of a small engine?
KIS- Keep It Simple, Always check the easy things first. If an engine suddenly
stops running, chances are the solution is an easy fix.
We must know how to use specialty tools like:
• Micrometer- measuring tool used to check components for wear
• Telescoping Gauge- t-shaped tool used to measure cylinder wear
• Plastigage- threadlike material used to measure connecting rod bearing
clearance
• Torque wrench- used to tighten fasteners to specific tolerances
• Ring-compressor- used to compress piston rings for piston installation
• Flywheel wrench or strap wrench- used to hold flywheel to remove nut
• Flywheel puller- used to pull stuck flywheels
• Spark tester- used to safely test spark
• Spring compressor- used to compress valve springs
• Feeler gauge- used to check clearances
• Valve grinder- used to grind and lap valves
4) What do we need to know to be able to establish troubleshooting techniques using
sequential procedure?
1. Check for spark.
2. Check for fuel.
3. Check for compression.
4. Do not disassemble engine unless absolutely necessary!
F. Evaluation:
Students will properly identify all of the small engine specialty tools on a written
exam in the classroom and an oral group quiz in the lab. Students will also be
expected to be able to briefly describe the functions small engine specialty tools in
essay format on the written exam. Students will be required to troubleshoot and
diagnose a small engine in groups and generate a plan of repair to present to their
classmates. Students will then repair the engine correctly to running condition.
Students will also be responsible for listing and differentiating between different
troubleshoot and repair scenarios by explaining their answers on a worksheet to
be turned in on the day of the written exam. Students will have a series of other
worksheets provided to them on the first day of this unit which are also due the
day of the written exam.