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How to Repair Small Engines

How to Repair a Small-Engine Ignition System
©2006 Publications International, Ltd. Here are the parts of a typical flywheel magneto.

An ignition system in a small engine produces and delivers the high-voltage spark that ignites the fuel-air mixture to cause the combustion. Some small engines require a battery to supply electrical power and the ignition spark. Others develop the ignition spark using a magneto.

A small engine ignition includes the ignition controller (mechanical-breaker, capacitor-discharge, or transistor-controlled), spark plugs, flywheel, and wiring. Servicing the ignition system of your small engine depends on which types of components it has. Below are step-by-step instructions for servicing ignition systems found in modern small engines.

Servicing Nonbattery Ignition Systems

A magneto applies magnetism to supply electricity in ignitions where there is no battery. The magneto is turned by the crankshaft, which rotates when the manual recoil starter is pulled. Here's how to service a non-battery ignition system:

Step 1: Service magneto.

Step 2: Service ignition controller.

Step 3: Service spark plug(s).

Step 4: Service flywheel.

If you need to service a magneto, use the following directions:

Step 1: Remove the magneto cap and clean surfaces with a small, clean paintbrush. Wipe any excess oil away.

Step 2: Service the ignition controller (mechanical-breaker, capacitor-discharge, or transistor-controlled) as described at the bottom of this page. 

Servicing Battery Ignition Systems

A battery is a device containing electrical cells that produce and store direct current. Batteries used to start and operate small engines typically store 6 volts or 12 volts. Here's how to service a battery ignition system:

Step 1: Service battery and charging system.

Step 2: Service ignition controller.

Step 3: Service spark plug(s).

Step 4: Service flywheel.

If you are interested in servicing a battery and charging system, here's how:

Step 1: Use a voltage tester to verify that the battery is storing sufficient voltage. Each cell develops approximately 2 volts (1.9 to 2.1 volts). A 6-volt battery testing lower than 5.7 volts or a 12-volt battery lower than 11.4 volts should be recharged to rated voltage or higher.

Step 2: If the battery is conventional (has three or six caps on top), use a hydrometer to check the density of the liquid electrolyte in each cell. The density or specific gravity should be between 1.26 and 1.28 at room temperature.

If it's below 1.25, recharge the battery. The difference in specific gravity between any two cells should be no more than 0.05. Note that a sealed maintenance-free battery cannot be tested in this way.

Step 3: Clean battery terminals using a small amount of baking soda and a stiff wire brush. Clean battery cables the same way.

Step 4: Inspect the cable insulation for corrosion and breaks; replace as needed.

Servicing Mechanical-Breaker Ignitions

For many years, mechanical-breaker ignition systems were the most popular of all ignition systems. The high voltage electricity from the coil is turned on and off using contact points and a condenser. The spark must be correctly timed to reach the spark plug at the exact moment when the piston is at the top of its travel and the fuel-air mixture is fully compressed. Here's how to service a mechanical-breaker ignition:

Step 1: Remove the cover from the stator plate to expose the breaker points and condenser.

Step 2: Manually turn the crankshaft until the high point of the cam lobe opens the contact points. Inspect the points for uneven wear or damage. If necessary, replace the breaker points and condenser.

Step 3: Slightly loosen the points setscrew and place the appropriate thickness gauge between the two contacts. (Check your owner's or service manual for the correct gap.) Move the points setscrew until the thickness gauge is touching both contacts but can be withdrawn without moving them.

Step 4: Tighten the points setscrew.

Step 5: Check the points gap with the thickness gauge again. Tightening the setscrew may have changed the gap.

Step 6: Clean the points with lint-free paper to remove any oil left by the thickness gauge.

Note: Some mechanical-breaker ignitions can be set using a dwell meter. If you have a dwell meter, refer to the unit's operating instructions and the ignition specifications to determine what dwell angle setting is correct and how to set it.

©2006 Publications International, Ltd. To adjust a mechanical-breaker ignition, loosen the points setscrew and use  a thickness gauge to check the gap between the breaker contacts.

Timing Mechanical-Breaker Ignitions

To work efficiently, the spark must be delivered to the combustion chamber at the exact moment that the piston is at or near TDC (top dead center). Here's how to time the ignition spark:

Step 1: Loosen the adjustment nut(s) on the stator so it can be turned.

Step 2: Disconnect the coil lead wire from the points.

Step 3: Connect a lead of the continuity tester light or ohmmeter to the breaker point terminal and the other to the housing or a ground location.

Step 4: Rotate the stator until the light or ohmmeter indicates that the points have opened the circuit (light off or resistance high).

Step 5: Tighten the adjustment nut(s) on the stator without moving it.

Step 6: Reconnect the coil lead wire to the points.

Servicing Capacitor-Discharge Ignitions

Capacitor-discharge ignitions (CDIs) store and deliver voltage to the coil by way of magnets, diodes, and a capacitor. The mechanical points of a breaker point ignition are replaced with electronics. The only moving parts are the magnets on the flywheel. That's why this system is sometimes called a breakerless ignition.

Because there are no breaker points on this system, there is no requirement for timing. However, the trigger module performs the same function as points. There must be a specific gap between the trigger module and the flywheel projection. Refer to your owner's or service manual for specific steps to setting this gap. The typical steps to setting the trigger module gap on a CDI system are the following:

Step 1: Remove the lead from the spark plug to prevent starting. Ground the spark plug lead by attaching it to the shroud.

Step 2: Rotate the flywheel so the projection is aligned with the trigger module.

Step 3: Loosen the trigger module adjustment screw(s) and insert a thickness gauge of the correct thickness (typically 0.005 to 0.015 inch) in the gap.

Step 4: Move the trigger module until it touches the thickness gauge, making sure the projection and module surfaces are parallel.

Step 5: Tighten the trigger module adjustment screw(s) and replace the spark plug lead. 

©2006 Publications International, Ltd. Capacitor-discharge ignitions and transistor-controlled ignitions look the same. Check the engine's owner's manual for specific information on servicing and repair.

Servicing Transistor-Controlled Ignitions

A transistor-controlled ignition (TCI) uses transistors, resistors, and diodes to control the timing of the spark to the engine. Because it has no moving parts, it too is called a breakerless or solid-state ignition. Most TCIs require no service. However, to ensure long-term performance, perform the following inspection on a regular basis.

The TCI controls a voltage of up to 30,000 volts for delivery to the spark plug. Be extremely careful when working around the TCI, as you can injure yourself or the ignition system with high voltage. Use the following steps to service a transistor-controlled ignition unit:

Step 1: Remove the lead from the spark plug to prevent starting. Ground the spark plug lead by attaching it to the shroud.

Step 2: Check the TCI unit to make sure it has not been damaged. Sensitive electronic components are mounted on a printed circuit board within the box and can be damaged by force or by excessive heat.

Step 3: Check all leads to and from the TCI unit to make sure they are tightly connected and that insulation is not cut or frayed.

Step 4: Visually inspect the magnet and ignition coil mounted next to the flywheel. Look for damage to the end of the magnet or the edge of the flywheel.

Servicing Spark Plugs

The spark plug in a small gas engine must withstand high voltage, high heat, and millions of ignitions during its life. A new spark plug requires about 5,000 volts of electricity to jump the gap. A used spark plug may require twice as much voltage to function. So servicing the spark plug is important to your engine's operation. Here's how to service a spark plug:

Step 1: Disconnect the lead wire from the top of the spark plug.

Step 2: Using the appropriate spark plug wrench, loosen the plug from the cylinder head. Before removing the plug, clean debris from around the spark plug base.

Step 3: Note the electrode's appearance. Excessive buildup can mean incorrect fuel-air mixture, incorrect carburetor adjustment, weak spark voltage, or poor air cleaner maintenance, among other causes.

Step 4: Clean the spark plug surface with a soft cloth and the electrode with a wire brush or spark plug cleaning unit. If the electrode is worn or damaged, replace the spark plug with one of the same size and heat range to avoid any damage to the engine.

Step 5: Using a feeler gauge, set the gap on the spark plug electrode to the manufacturer's recommendations.

Servicing Flywheels

The flywheel on a small gas engine is a simple part that requires little service. The most important part of servicing a flywheel is to inspect it for damage periodically. Here's how to service a flywheel:

Step 1: Remove the lead from the spark plug (to ensure that the engine doesn't start), then rotate the flywheel by hand and inspect it for wobble and obvious damage. Check edges and cooling fins, looking for cracks and missing pieces that can make the flywheel -- and the engine -- rotate out of balance.

Step 2: To inspect the inside of the flywheel, use a flywheel puller or a knock-off tool to remove the flywheel from the end of the crankshaft.

Step 3: Inspect the magnets on the inside of the flywheel, if so equipped. Wipe all surfaces clean, removing rust, oil, and debris.

Moving parts of a small engine can wear out prematurely if they aren't lubricated. Learn tips about how to lubricate the engine, using oils and additives, in the next section.

©2006 Publications International, Ltd. You may need a special tool called a flywheel puller to remove the flywheel.

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