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.


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.


Electronic ignition module is sealed plastic unit that contains both ignition coil and electronic ignition components. These modules require almost no maintenance, most modules can’t be adjusted in any way. In some engines with electronic ignition modules, possible to adjust air gap. This procedure performed same way we described for non electronic magneto ignition system.

Good idea to perform visual check on module to make sure wiring and terminals in good condition. Can’t replace components inside module. If problems detected, electronic module should be replaced. Units inexpensive, readily available from parts suppliers.


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.


Points and condenser used for many years on power equipment. In newer engines, these components replaced by electronic ignition systems. Breaker points flawed in that they eventually wear out and fail. Electronic ignition systems use diodes, transistors, SCRs in place of mechanical switching components, so they last for very long time.

Electronic ignition in outdoor power equipment will almost always use electronic ignition module and flywheel magneto. Electronic ignition module is sealed plastic unit, often black, that contains ignition coil, electronic trigger switching devices, and usually a capacitor. Module usually mounted on a bracket very close to outer edge of flywheel. A spark plug wire leads from module to spark plug. You’ll also see small grounding wire on module. End of grounding wire usually connects to grounding terminal on mounting bracket for kill switch.

Electronic ignition systems have no moving parts, except for flywheel containing magnets, so performance of system won’t decrease through attrition. Electronic ignition modules very resistant to moisture, oil, and dirt. Are very reliable, don’t require adjustments, have very long life spans. Provide easy starting, smooth power during operation. Also generally quite inexpensive.

Two basic types electronic ignition configurations:

1)The capacitor discharge (CD) ignition system
2)The transistor controlled (TC) ignition system


Electronic system most often used is capacitor discharge ignition system. Basic components of CDI system configured in several ways. Usually all components contained in sealed module, however in certain engine applications, CDI components have different arrangements of wiring and parts. No matter the arrangement, all CDI systems operate much the same.

Consider typical CDI system. System contains two coils, triggered by magnets in flywheel. Larger coil is charging coil, second smaller coil called the trigger coil. Trigger coil performs function of breaker points in system, controls ignition timing.

As flywheel rotates past charge coil, energy produced in module. Capacitor stores this energy until it’s needed to fire spark plug. As flywheel magnet rotates past trigger coil, a low voltage signal produced. Signal causes electronic switch to close, which allows energy in capacitor to pass through switch to transformer. At transformer, voltage increased to 25,000 volts. High voltage travels through high tension lead to plug, creating spark at electrode to ignite mixture.

Consider another type CDI system. Charging coil and trigger coil mounted underneath flywheel. Ignition coil, other components located away from flywheel, closer to plug. This type system sometimes called externally mounted CDI system. This type CDI may be seen in motorcycle ignitions, but not usually in lawn mowers and other such equipment.

Secondary power source for this CDI system is stored charge in capacitor. Magnets in flywheel used to energize both charging coil and trigger coil. As flywheel turns and magnet passes over charging coil, one cycle alternating current produced. Current is passed to group of four diodes, which rectifies it to direct current, passes it on to capacitor. Capacitor charges to full capacity. At this point, ignition system prepared to fire.

Flywheel continues to turn. As magnet in flywheel passes trigger coil, brief pulse of current generated. Current pulse passed to gate terminal of SCR. SCR turns on, allows capacitor to discharge stored charge to ignition coil. Remember that ignition coil is transformer. As current from SCR rushes into ignition coil, strong magnetic field builds up in coil’s primary winding. Magnetic field in primary induces strong voltage in secondary, voltage discharges across plug gap.


In transistor controlled ignition system (TC), transistors used to perform trigger switching. Components of transistorized system generally contained in one small unit mounted directly to ignition coil. Because of construction, no test points to monitor in event of system failure.

A transistorized system operates by controlling flow of electricity to primary coil. Two transistors contained in control unit. One transistor used to supply electricity to primary coil. When voltage level in primary reaches certain level, second transistor turns off first. This causes magnetic field around primary coil to collapse, creating high voltage across secondary coil. Voltage then discharged across plug.