If magneto has electronic switching components, little to check. Just general visual inspection of wiring and terminals. In older systems with breaker points, points often wear out and fail. Systems that contain points must be inspected carefully. Points can be located in two locations in engine. In most two stroke engines, will be under flywheel. In four stroke engines, may be under flywheel or timing cover.

If points and condenser under flywheel, first step to remove flywheel. Flywheels loosened by special tools, either a knock off tool or a flywheel puller. Type of tool will depend on engine’s construction. When flywheel removed, may as well inspect it. Check for rusting, corrosion, broken fins. If flywheel has broken fin, entire flywheel should be replaced. Test magnets in flywheel by placing metal socket against each one. Socket should stick to magnet when you shake flywheel. If magnets have lost their power, magneto won’t work; flywheel must be replaced.

With flywheel removed, breaker points cover exposed. Cover protects points and condenser from liquids and dirt. Cover can be removed by removing two retaining screws, lifting off cover. If points and condenser behind timing plate cover in upper cylinder head area, remove cover screws and cover.

Will usually find great deal wear, pitting in contact area of points. In first stages point contact wear, points begin to pit. Next, pit will increase in size. Material from one contact may be deposited on second. Pitting can be so bad that contacts stick or weld together.


If points worn, can be easily replaced. Procedure usually provided in service manual for engine. General procedure would include following:

1)Locate retaining screw. Remove screw and points. Note position of points so can reinstall them correctly.
2)As you lift out points, you’ll see wire from primary side of coil and second wire from condenser attached to points. Remove nut that holds these wires on points and remove wires.
3)Locate, remove condenser retaining screw. Remove screw and lift out condenser. After screw removed, condenser removing tool can be used to grip and remove condenser. Note position of condenser when you remove. Note that in certain systems, condenser will actually be part of stationary contact in points assembly.
4)Before installing new points and condenser, compare old parts to new. Make sure are same size, and that mounting holes in same locations.
5)Install new condenser, tighten its retaining screw.
6)Connect wire from condenser and wire from ignition coil to points and tighten down retaining nut.
7)Mount points inside enclosure.


Distance between the breaker point contacts called the point gap. Is precision measurement determined by manufacturer, like spark plug gap. Point gap must be measured to ensure proper functioning of points.

A blade type feeler gauge can be used to measure point gap. Determine proper gap width by checking service manual for engine. Gap will normally be in range of 0.020 inch to 0.030 inch. Rotate crankshaft of engine until points at full open position. Find feeler gauge blade that matches width specified in manual. Insert blade between point contacts. If necessary, can combine two or more blades to get specified width. If point gap width correct, should feel slight drag between contact points and gauge blade. If gap too large or too small, should be adjusted.

To adjust point gap, loosen point gap adjusting screw, move points until proper gap reached. In some engines, can insert screwdriver into slot in points and twist screwdriver to open or close points as necessary. Manufacturers recommend different methods for setting point gap in their engines. May involve use of tool called dial indicator or other instruments. Techniques will be described in detail in service manuals.


Air gap of magneto is small distance between rotating magnets and armature of ignition coil. Air gap another precision measurement determined by manufacturer. Ideally should be as small as possible, because closer magnets are to armature, greater the amount of current induced in primary. Magnets should never come in contact with armature during rotation of flywheel or rotor. In some engines, magneto air gap can be adjusted. Not possible to adjust air gap on all engines, particularly those that contain electronic components.

When ignition coil mounted outside flywheel, air gap easy to measure and adjust. Can measure gap with air gap gauge. Air gap gauge is index like card made of plastic or cardboard, manufactured to precise thickness. May also use nonmagnetic feeler gauge to measure magneto air gap. Won’t get accurate measurement if use steel feeler gauge because magnets in flywheel will pull on gauge blades. Instead use brass gauge to make measurement.

If magneto air gap incorrect, can be adjusted by following steps:

1)Align flywheel magnets with ignition coil armature.
2)Place proper size gauge between coil and flywheel.
3)Loosen armature mounting bolts. Magnets will pull armature toward flywheel until it rests against gauge.
4)Retighten armature mounting bolts.
5)Remove gauge, check clearance. Rotate flywheel a few times to make sure no part contacts armature.

If flywheel does contact armature, flywheel may be warped or loose, flywheel key or keyway may be worn, or crankshaft may be bent. If armature has no provision for adjusting air gap, any of these conditions may be cause of improper air gap.

If coil located under flywheel, following steps should be taken to adjust magneto air gap:

1)Remove flywheel.
2)Place piece of electrical tape on inside rim.
3)Replace flywheel (finger tight).
4)Rotate it 10 to 12 times (removing spark plug will make engine easier to rotate by hand).
5)Remove flywheel and examine tape.
6)If tape scuffed, air gap clearance too small.
7)If tape isn’t scuffed, add another strip, repeat test. If tape still not scuffed, air gap clearance too great.
8)Adjust position of armature, until air gap clearance such that one layer tape isn’t scuffed but two layers are when flywheel rotated.


Are different types switching devices. Two basic types trigger switching devices used in small engine. Some use set of electrical contacts called breaker points and condenser as switch. Other systems use electronic components to perform switching.

Breaker points are mechanical contacts that stop and start flow of current through coil. Points usually made of tungsten, a very hard metal with high resistance to heat. One breaker point stationary or fixed, the other movable. Movable contact is mounted on spring loaded arm. Spring pressure used to hold points together.

When breaker points touch, ignition circuit complete and primary winding of transformer energized. When end of spring loaded movable point pressed, contact end moves apart from stationary point. Circuit thus opens and flow of current stops. Each time points move apart, spark plug fires.

Movable point is moved to open position by turning lobe or plunger. Lobe may be located on flywheel or end of camshaft. Plunger device operated by crankshaft or camshaft, will move in and out to press on movable point. After spark plug fires, spring mounted under movable point returns it to original position. Cam or plunger responsible for timing of spark.

Another important component of breaker points system is condenser. Each time breaker points touch, current flows through them. Unless current flow controlled, a spark or arc will occur across points as they move apart. This would burn breaker points up. They would also absorb most of magnetic energy in coil, and prevent it from producing high voltage in secondary winding.

Condenser used to control current as it flows through points. As soon as points begin to separate, condenser absorbs current so it can’t jump between points and make spark or arc. When plug fires, condenser releases current back into primary circuit.

Condenser is type of capacitor. Capacitor is electrical component that can store charge. When current applied to condenser, condenser absorbs current and stores it. As points open, capacitor absorbs electricity created by collapsing magnetic field in primary winding. Condenser thus prevents electricity from jumping gap between points. When magnetic field collapses in secondary, plug fires. At same instant, condenser releases charge back into primary winding.

Condenser is cylinder shaped component made of two aluminum foil strips wound together, separated by insulating paper. One aluminum strip has electrical lead connected to it. Wound strips then inserted in cylindrical metal case. Grounding connection attached to outside of case. In ignition circuit, condenser connected across or parallel to points.

Breaker points and condenser work together to form switching device. Breaker points and condenser combination used in both magneto and battery systems. Other type of switching device in small engine applications is electronic switch. Solid state electronic components used to turn current flow to primary winding on and off. Electronic switch eliminates breaker points and condenser.


In magneto, permanent magnets installed in engine’s flywheel or rotor. Coil mounted in stationary position near flywheel. When flywheel spins, magnets induce voltage in primary winding of coil. Coil in magneto system may also be located on bracket at side of flywheel.

Consider typical flywheel in magneto system. Flywheel often cast of aluminum alloy. During casting process, two or more magnets encased within aluminum. These magnets will pass coil as flywheel rotates.

Position of magnets on flywheel very important. For magneto to work properly, voltage must be generated in primary winding of coil at correct moment of flywheel’s rotation. To generate voltage at exact time needed, magnets in flywheel must be properly aligned. This means flywheel must be located in proper position on crankshaft.

Flywheel held in position on crankshaft by bar of soft metal called flywheel key. Flywheel key inserted into matching slots cut into crankshaft and flywheel. Together these slots called the keyway. Flywheel key holds crankshaft and flywheel in alignment. In modern engines, flywheel held to crankshaft with shear key. This key will break off or shear if flywheel becomes jammed. Shearing action of flywheel key disengages flywheel from crankshaft and stops engine.

For magneto to work, coil must be mounted in stationary position near flywheel. All newer engines that use flywheel magneto ignition will contain electronic ignition coil. Is a small air gap between flywheel’s edge and coil. Air gap is important specification in ignition system. Engine manufacturer will determine proper width of gap in thousandths of an inch. Gap must be exact for magneto system to work properly. This specification must be checked when servicing ignition system.

The breaker points system is located underneath flywheel. Remember that ignition coil is transformer containing two windings of conductor wire. Primary winding consists of about 150 turns fairly heavy copper wire, while secondary consists of about 20,000 turns of very fine copper wire.

Assume flywheel turning clockwise. Flywheel will be turning at high rate of speed. As flywheel turns, magnet mounted near edge of flywheel begins to pass by coil. As magnet passes coil, magnetic lines of force from magnet move into armature of coil. Magnetic lines of force move from north pole of magnet through armature and back out south pole of magnet. The magnetic field induces voltage in primary winding of transformer.

As flywheel magnet continues its rotation, magnetic lines of force from magnet suddenly move through armature in opposite direction. This happens because sudden change in position of north and south poles of magnet. Change in direction of magnetic lines of force cause current to flow in primary winding of transformer. Primary winding connected to breaker points. Since points closed at this time, current flowing through points.

Voltage in primary winding will induce low voltage in secondary due to mutual inductance. Voltage in secondary, at this time, still too low to jump gap of spark plug. At this point, turning cam lobe in points assembly begins to open points. As points separate, current flow in primary circuit broken. Magnetic field around primary collapses through secondary winding of transformer. Any current left in primary absorbed into condenser. Absorbing action of condenser prevents remaining voltage in primary circuit from arcing across points.

As magnetic field collapses through secondary, high voltage in secondary induced. At exact same time, charge absorbed, stored in condenser flows back into primary. This discharging action helps increase voltage in secondary. High potential of voltage induced in secondary causes current to flow through spark plug wire, arc across plug gap.

After high voltage in secondary winding released as spark, flywheel continues rotation until magnet positions itself by coil again, and process repeats. Actions described here occur very very quickly. Engine may require as many as 1,800 sparks per minute to operate.


In battery system, battery used to provide power to ignition coil instead of magneto. System, however, contains same type of switching components, either points and condenser or electronic switching components, and spark plug. Battery used in this type system is lead acid storage battery, similar to type used in automobile. Battery in small engine application generally much smaller. Battery may also power lights, horn, and other accessory circuits.

Typical lead acid storage battery made up of individual compartments called cells. Each cell made up of a series of lead plates. Spaces between plates filled with electrolyte solution. Electrolyte made from sulfuric acid diluted with water. Each cell produces approximately 2 volts when battery fully charged, so a 12 volt DC battery will contain 6 cells. Acid used in batteries dangerous, can cause burns or destroy clothing.

Storage battery in battery ignition system has total output 12 volts direct current. Current produced by battery measured in ampere hours (Ah). In battery ignition system, a generator, alternator, or coils within flywheel may be used to recharge battery as engine operates.

Consider typical battery powered ignition system that uses points assembly for triggering. Battery provides voltage to energize primary winding of coil. Voltage to coil switched on and off by ignition switch. Switch often operated by key in garden tractors. In some garden tractors and riding mowers, ignition switch may have multiple sets of contacts to engage starter solenoid and other options or accessories.

When ignition switch turned on, switch contacts close and ignition circuit closes. When circuit closes, power from battery passes through ignition switch and through primary winding of ignition coil. Opposite end primary winding connected to points and condenser. Points assembly, secondary winding, spark plug all operate in exactly the same way as in magneto system. Only difference that battery energizes primary winding of coil. When ignition switch turned off, switch contacts open, flow of power from battery to primary winding stopped and, therefore, engine stops.


Many pieces outdoor power equipment contain both magneto and battery. In such machine, magneto is power source for ignition coil. Battery used to operate electric start and accessory circuits.

Electric starter is small DC motor that has movable gear on its output shaft. When key turned in ignition switch, circuit from battery to starter motor closed, and current flows from battery to motor. Starter motor’s armature turns. When armature turns, gear on armature pushes outward, mates with gear teeth of flywheel. Turning gear on motor’s armature causes flywheel to start turning. When flywheel starts turning, gear on motor armature retracts. Since this type system also contains magneto, when flywheel starts turning magneto begins working. Only difference in this system that key used to start machine instead of rope.