Briggs and Stratton engines, like all small engines, require fuel, air and fire for proper operation. Fresh gasoline must be correctly fed into the combustion chamber. Air is needed for that combustion and is cleaned by an air filter that sits atop the carburettor. Fire is the spark that ignites the fuel and air mixture. The ignition system generally consists of a spark plug, the spark wire and a magneto or coil. The spark plug sits directly above and inside the combustion chamber. The spark wire conducts the high voltage electrical power that is generated by the magneto or coil. The coil is positioned next to the flywheel and generates electricity by a permanent magnet on the rotating flywheel. With each pass of the magnet, one revolution of the flywheel, a high voltage electrical signal is made by the coil.
Remove the flywheel cowling using an end wrench. Depending on the model of engine, there are three bolts that are either a 7/16 or 1/2-inch sized bolt head. Lay the cowling and bolts to one side.
Note there are only two electrical connections to the coil. One end is the spark wire which is attached to the end of the spark plug. Remove the rubber hooded wire from the top of the spark plug.
Use the 1/4-inch nut driver to remove the small bolt that holds the ring connector to the metal frame of the engine. The ring connector is attached to the other end of the coil. Observe that the wire will be a bare copper wire with a varnish-like coating.
Place the leads in the volt ohmmeter. Take the red lead and plug it into the connector marked "ohms." The black lead will go into the connector marked "common." Switch the meter to the ohms position on the power dial. Touch the leads together. The meter short read "zero" ohms or a direct short.
Touch the red lead to the connector under the rubber bonnet of the spark wire. Connect the black lead to the ring connector on the other end of the coil. The meter should read two to five ohms. If the meter reads "zero" or a direct short the coil maybe bad. The coil will also need replacing if the meter indicates an "infinite" ohm reading or a resistance greater than 1000 ohms.
Reattach the small ring connector to the frame of the engine. Touch the black lead to the frame and the red lead should still be in contact with the connector in the spark wire. Turn the meter to volts. The red lead should also be placed into the "volt" connector on the meter.
Move the flywheel to a position where the permanent magnet on the flywheel sits to one side of the coil. Set the meter in a position where you can read it without holding onto it. Turn the flywheel back and forth where the magnet will pass in front of the coil. It may take two hands to perform this task. As the magnet quickly passes in front of the coil face it will generate a small voltage through the coil. The meter should read some type of voltage.
Check the condition of the spark plug if the coil passes all the above tests. The insulating porcelain on the plug could be cracked. This will short out the high voltage power from the coil. Check the gap between the coil face and the magnet. Use the engines specification for this air gap. Various sized engines will have different air gaps. Use a feeler gauge for this critical measurement.
Tips and warnings
- Check the condition of the spark plug if the coil passes all the above tests. The insulating porcelain on the plug could be cracked. This will short out the high voltage power from the coil.
- Check the gap between the coil face and the magnet. Use the engines specification for this air gap. Various sized engines will have different air gaps. Use a feeler gauge for this critical measurement.