Village Science - Teacher Edition


Magnetos & Spark Plugs

Teacher Edition Contents

Skill, Tools, & Craftsmanship

Cutting & Drying Fish
Nails, Pegs, & Lashings
Falling Trees &
     Small-Scale Logging
Chainsaw Clutch & Chain
Ice Pick


Wood Stoves
Wall Tents
Insulation & Vapor Barriers
Gas Lamps & Gas Stoves


Piloting A Boat
Boat Design
Magnetos & Spark Plugs
Outboard Motor Lower Unit

Outboard Motor Cooling System
Snowmachine Tracks
Snowmachine Clutch
Winter Trails


  1. Collect as many different kinds of spark plugs as you can find in the village. How many different kinds do you find? How many different companies are represented by the plugs? What differences do you notice among them? How many different kinds of engines do these plugs represent?

    The differences are numerous. The diameter and length of the threads that go into the cylinder heat, the diameter and length of the porcelain, the electrodes etc. A collection of 10-15 different ones is small.

  2. Compare the length of the plugs, the length exposed within the cylinder, the length of the threads, and the diameters. Why do you think there are such differences? Can you find some plugs that are golden brown, some black, and some that burned too hot? (You may not, as the golden and white ones might be still in machines.)

  3. What are the differences in the identification numbers of hot and cold plugs from the same manufacturer. If you can’t tell by looking, a manual will tell the difference.

    Snowmachine plugs will best illustrate this.

  4. Find old flywheels. Test the magnets for strength. Are they strong or weak magnets?

    Fairly strong.

  5. Get an old and a new spark plug of the same kind. Put the plug wire of an engine (chainsaw is easiest) on each plug. Hold the base of the plug against the cylinder of the engine, and crank the engine over. Do you see a difference in the color of spark in the new and old plug? (It is hard to see the spark in a bright location.)

    If the old plug sparks at all it will probably be yellow or red. The new one should be blue or white. As stated above, do this in a dark room, not in direct sunlight. Be careful. You can get a good shock from this. The person holding the plug to the cylinder head should not touch any other part of the engine.

  6. Find a plug that will not fire in the above manner because of carbon and dirt. Carefully clean the center post with a hairpin, or other slim object. Can you clean it well enough to give a hot blue or white spark?

    Now is the time to learn. In the middle of a snow or rainstorm is not the time to experiment.

  7. Close the gap on an old plug, and test it against the cylinder. Did the color of the spark change when the gap was made smaller?

    It should improve a little, but this doesn’t always work.

  8. Look up the recommended spark plug gap for three or four different engines. The recommended gap should be in the manual for the machine. Pick some high and some low compression engines. Why do you think there are some differences in the recommended gaps?

    High compression engines have more resistance to the spark jumping the gap. However, the manufacturers know this, and make the coil stronger. The gaps should all be about .0020-.0025

  9. Find out about sandblasting spark plugs. Ask how long a sandblasted plug will stay clean and why.

    Sandblasting works for a short while, but the rough surface created will accumulate carbon fairly soon.

  10. Put the end of a fouled spark plug in the flame of a propane or Coleman stove until it turns red hot. Carefully let it cool. Test the spark before and after this. What difference do you see in the spark? Why do you think this is so?

    This cleans the plugs, as it burns the carbon from them. They won’t work as long as a new plug, but will get you home if you are stuck out in the woods.

  11. Find a coil that people say is bad. Is there anything visible to indicate that it doesn’t work well?

    The sad answer is… “no”. There is nothing visible about a coil that would indicate it’s functioning.

  12. On a working engine, pull the wire that goes from the magneto to the coil. While holding the wire, ground your hand against the cylinder and pull the starter rope. Put that wire back and pull the wire from the spark plug. Put a screwdriver handle up the spark plug cap and again ground your hand against the cylinder. Pull the starter rope again. Do it slowly! Can you feel the difference in voltage? (You should!) This will be uncomfortable, but shouldn’t hurt unless you pull very fast.

    The voltage coming from the magneto to the secondary coil is hardly noticeable. The voltage from the secondary coil to the sparkplug is strong. If you don’t pull the engine fast at all, it will give you a little jolt, but won’t hurt. Don’t touch the engine with any other part of your body other than the hand held against the cylinder. The spark will go through your hand, and not thru your body. This might sound a little dangerous, but when you are stuck out in the woods, this is one of the ways to tell if the spark is strong enough. It is better to learn how to do it safely.

  13. Ask around the village to see if anyone knows the difference between a generator and an alternator. What is the difference?

    A generator has a permanent magnet in the armature. An alternator has an induced magnet in the armature. An alternator can generate more electricity, particularly at low rpm’s, but it needs some electricity in the battery to induce the magnetic force. That is why a car with a completely dead battery cannot be jump started by pushing. All new cars have alternators, and they need a little electricity to activate the magnet.

  14. Draw a cylinder whose timing is too advanced. Draw one whose timing is too slow. Look in an owner’s manual of a four-cycle engine and find how many degrees before top dead center (BTDC) the timing should be set.

    The drawing is in the text.

Student Response

  1. Make a simple drawing of the parts of the spark system of an outboard motor from magneto to spark plug.

  2. Where is the electricity generated in an engine?

    In the magneto

  3. Where is the voltage increased?

    In the secondary coil

  4. What does the spark plug do in an engine?

    Ignites the fuel

  5. Draw and label three cylinders, one firing too soon, one firing too late, and another firing at the proper time.

  6. Explain why a spark might jump the gap in open air, and not in the cylinder.

    There is more resistance in the cylinder. There are more air molecules in the way.

  7. What colors indicate the hottest sparks?

    Blue and white

  8. What colors indicate the weakest sparks?

    Red and orange

  9. What two things could keep a sparkplug from firing well?

    The gap is not set properly

    There is carbon on the plug, shorting it out.

  10. List five things that could make the sparkplug black with carbon?

    Fuel mixture too rich

    Spark is too weak

    Wrong kind of 2 cycle oil

    Too much oil in the gasoline

    The plug doesn’t retain enough heat

  11. Draw a picture showing how a dirty plug allows the spark to ground out, not jumping the gap.

  12. What kind of plug should be used in an air cooled engine during cold weather? Why?

    A hot plug. It can thoroughly burn the fuel on it’s surface. A cold plug will carbon.

  13. Why would an outboard be able to use one type of plug all the time, and a snow machine need different plugs in different seasons?

    An outboard is cooled by water that is a fairly constant temperature. A snowmachine is cooled by air that might be from -50° to v40°F.

  14. List five things that could cause the voltage to a sparkplug to be weak.

    Dirty or worn magneto coil

    Improper distance between the magnet on the flywheel and magneto coil.

    Broken or dirty wire connections.

    From the magneto to the secondary coil

    From the secondary coil to the ground

    From the coil to the ground.

    Secondary coil shorting out

    Dirty or cracked spark plug.


  1. The voltage generated by a magneto is 50 volts. The coil increases this to 15,000 volts. If the magneto is fixed so that it now generates 75 volts. How many volts will the coil produce?

    22,500 volts

Questions or comments?
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