Set the blower on a flat workbench.
Remove the spark plug wire from the spark plug located on the top of the engine, remove the spark plug with the spark plug wrench. Install the new spark plug.
Plug the lead from the spark tester into the spark plug wire. Attach the clip from the spark tester onto the metal end of the spark plug.
Turn the knob on top of the spark tester and adjust the gap to 4mm. Pull the starter recoil rope rapidly while watching the window in the spark tester. The spark should be a strong, steady, blue spark; if the spark is weak or there is no spark at all, the coil needs replacing.