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How do I Troubleshoot a Stihl Fs45 Trimmer?

mower image by Gintautas Velykis from

The Stihl Fs45 Trimmer is one of their most popular grass and weed trimmers. It is also the lightest weight model in the Stihl HomeScraper Series and has a curved shaft that helps with trimming around shrubs and fences. The Fs45 Trimmer is designed for occasional use, but as with most garden tools and appliances with multiple moving elements, normal wear and tear can occur. Before taking the unit in for a repair, you can troubleshoot a few steps at home.

Move the stop switch toward the rear of the trimmer if the engine does not start. Remove the spark plug boot then unscrew the spark plug and dry it off. Open the throttle fully and crank the engine. This should clear out the combustion chamber. Replace the spark plug and boot. Move the switch to the forward position and start the motor.

Clean the air filter if there is a loss of power to the unit. Push in the button to the filter cover and open the door. Wipe any dirt or debris away from around the outside of the filter. Remove the filter and blow it with compressed air or knock it against your hand to dislodge any dirt. Replace the filter and snap the cover into place.

Clean the spark plug if the trimmer is losing power. With the engine turned off, remove the spark plug. Wipe the plug clean. Adjust the electrode gap if necessary. The electrode gap is the space between the top of the spark plug and the thin strip of metal that curves above it. Replace the spark plug and boot. Make sure the boot is secure.

Turn the idle-speed screw clockwise if the engine stops while it is idling. Make sure that the cutting element is not rotating. Keep turning the idle screw until the engine runs smoothly.

Turn the idle-speed screw counter-clockwise if the cutting tool keep rotating while the engine is idling. Once the cutting tool stops turning, turn the screw another half inch to 1 inch counter-clockwise to secure.


Replace the spark plug after every 100 hours of use.

Do not pull the rope out further than specified. Make sure not to let the rope and handle snap back, rather, guide it back into place by hand.


A loose connection between the spark plug boot and the ignition wire connector could create arcing and fire.

Always check the mowing head for wear before replacing nylon wire.

The Stihl Fs45 is a high-speed cutting tool. Always wear protective eye wear.

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