A Husqvarna weed eater operates on the same principles as any other internal combustion engine. These engines require enough fuel to mix with the right amount of air and enter the combustion chamber at the right moment for the spark to ignite the mixture. Once the fuel gets ignited, the whole process continues in a self-feeding loop. However, if the Husqvarna is running poorly or won't start at all, something is interrupting the flow of fuel, air and spark.
Move the ignition switch on the Husqvarna into the "Off" position. Set the weed eater on a level workspace. Lift the rubber boot off the tip of the spark plug. Unscrew the spark plug and remove it from the cylinder.
Fit the test plug into the rubber boot. Clip the test plug to a grounded metal point on the engine cylinder. Wear work gloves and hold the test plug in place if necessary.
Switch the ignition into the "On" position. Pull on the starter rope and check for a spark on the test plug. If no spark is detected, or it's a weak yellow, service the ignition system.
Disconnect the test plug from the engine and boot. Pull on the starter rope several times to clear the chamber. Screw the compression gauge into the spark plug hole until it's finger tight.
Pull on the starter rope again until the gauge's needle stops increasing. Note the compression levels, which should be between 90 to 110 psi for a good engine. Service the compression and air systems if the reading is below this.
Unscrew the air filter box and remove it from the engine to access the carburetor. Pry the two fuel hoses off the end of the carburetor. Tilt the engine slightly and look for gas to come out of the hoses. If no gas flows out, service the fuel system.