How to Troubleshoot the Briggs & Stratton 18 HP Ignition System

Power off your lawn mower and open the front engine hood. Ensure the thin, black ignition wire is connected to the spark plug on the side of the Briggs & Stratton engine. Reinsert the ignition wire into the spark plug's rear if necessary.

Disconnect the ignition wire from the Briggs & Stratton engine's spark plug and replace the plug entirely if the mower still won't work. Insert the ignition wire into the replacement spark plug.

Trace the spark plug's ignition wire to the rear of the Briggs & Stratton engine assembly. It leads to the magnetic coil. There is an incremental gap between the magnetic coil and the circular flywheel piece directly behind it. This incremental gap is the air gap, and it must be spaced precisely to ensure the engine works and starts correctly. Measure the small air gap with a caliper and ensure it falls between 0.010 and 0.015 of an inch in width.

Loosen the bolts on the magnetic coil with a socket wrench until you can slightly readjust its positioning in relation to the flywheel. Make your adjustments and then remeasure the air gap with your caliper and ensure it falls between 0.010 and 0.015 of an inch.

Close the hood over the Briggs & Stratton engine.