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Small Engine Electrical

Most small engines use a basic electrical system for the sparkplug. The sparkplug is used for generating a small spark inside the combustion chamber. There must be an electrical path for the onboard transformer or magneto to generate this spark in order for the small engine to operate. Understanding this electrical system will go a long way in identifying any problems.

The Spark System
Residing alongside the engines flywheel is a small transformer or magneto. The flywheel is located on either the top or the side of the small gasoline engine. It is the portion of the engine that is directly connected to the pull starter rope. Imbedded inside the main flywheel is a permanent magnet. As this magnet passes by the transformer a small electrical current is generated. Most small engines when placed at full throttle will run at approximately 1800 rotations per minute (RPM). This means that the permanent magnet is passing by the transformer 1800 times every minute. One electrical wire from transformer is then directly connected to the sparkplug. The sparkplug is located under the thick black wire with a rubber coating. Another wire that is also connected to the transformer comes from a set of electrical contacts called the points.

The Points
The points are mechanically connected to the main shaft of the small engine. The same main shaft that powers the lawn mower blade or a pulley. The contacts open and close with each single revolution of the engine. So just as the transformer generates a spark for the sparkplug so does the contact open and close. In fact these two events are timed to each other. This ensures that the electrical spark is sent to the plug at precisely the correct time for the combustion of the gasoline in the small engine. The other side of the contact is typically connected to a small condenser or capacitor. This aids in storing a charge in case the transformer fails to generate a spark on any particular revolution.

The Complete Circuit
If we begin at the set of contacts you will see a small wire that is connected directly to the metal frame of the engine. That wire then goes through the set of contacts which open and close with each single revolution of the engine. Connected into the wire on the contact is another single wire from the condenser. These two wires form a single electrical connection. The contact which opens and closes has a wire that leads from it to one side of the transformer. This is the low power side. The continuous wire then runs through the transformer as a set of windings. This single wire is literally wound around a central metal core. Each winding increases the output voltage. Transformers contain hundreds of these wire windings.  The wire then runs from the high voltage side of the transformer and into the sparkplug. A bad set of contacts or a broken wire from this circuit can cause the engine not to run. Many times the small solid wire that runs from the contacts to the condenser can be broken under the wires protective insulation. The electrical connection that lays under the thick rubber hood of the sparkplug connector can be corroded. All of the basic electrical system should be checked if you are experiencing a small engine that does not operate.

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