How to Test a Small Engine Magneto

Small-engine magnetos generate an electrical voltage that in turn produces the spark for the spark plug of the combustion cycle. The magneto usually is located under the main cover shroud that protects the flywheel. Embedded in the flywheel is a large magnet. As the flywheel rotates, the magnet passes by the wire-encased magneto or transformer, and induces an electrical voltage into the copper windings. With each passage of the flywheel, a timed voltage is sent to the spark plug to ignite gasoline vapors in the combustion chamber. By following these basic procedures, you can test the magneto on most small engines.

Difficulty: Moderately Challenging

Instructions

Things You'll Need:

• End wrenches or socket set
• Volt ohmmeter

1. Remove the flywheel shroud with the end wrench or sockets. Generally, there are three bolts, measuring either 7/16 or ½ inch. One bolt will be located on the flat side of the metal cover and the remaining two bolts will on the round side.

2. The magneto will be the only part that has exposed copper windings. The magneto will have a large, rubber-coated wire coming from it and will be connected directly to the spark plug. Another smaller wire, the case ground, will be connected directly to the metal case of the engine body.

3. Pull the rubber-coated wire from the spark plug. Inside the rubber hood, there will be a round clip that fits over the end of the spark plug. This is the high-voltage side of the magneto. Remove the small bolt that secures the groundside wire to the engine case.

4. Be sure the ground wire does not touch the engine case, or a false reading will occur. Switch the ohmmeter to ohms, and touch the red probe to the ground wire and the black probe to the spark plug clip. The meter should register a direct short or "continuity." Remove the black probe from the clip and touch this to the engine case. The meter short read infinite ohms or a "wide open." If the meter registers any form of continuity, then the magneto has an internal short and should be replaced.

5. Reconnect the ground wire to the engine case. Switch the volt ohmmeter back to volts, and select the high-voltage reading. If the meter does not have the capability to read high voltage, then skip this action, because it may damage the meter. Touch the red probe of the meter to the spark plug clip and the black end of the probe to the engine case. Rotate the flywheel past the magneto a few times. The voltage meter should register a reading as the magnet on the flywheel passes by the magneto. If it does not, then the magneto may be weak.

6. Replace the flywheel cover, and pull the spark plug from the cylinder head of the engine. Insert the spark plug back into the rubber hood, and lay the plug against the metal frame of the engine. Pull the start cord, and observe the end of the spark plug. As the engine turns over, the spark plug should emit a bright white spark. If no spark is generated, then the plug will have to be replaced.

Tips & Warnings

• Change spark plugs as a regular part of annual maintenance for small engines.

References

• Briggs& Stratton Repair Instructions; Briggs & Stratton Corp.; 1975

[howarrd]

on 8/29/2009 excellent...well written step by step