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How to Troubleshoot a Stihl FS 55R Trimmer

By: Shawn McClain

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17 July, 2017

Siri Stafford/Lifesize/Getty Images

The Stihl FS 55 is a straight shaft trimmer and brush cutter designed to let you easily maneuver it around obstacles on your property. The FS 55 is run by a 1 brake horsepower (.75 kilowatt) engine that operates between 2,800 and 9,500 revolutions per minute. The "R" version of the FS 55 comes with a loop handle halfway down the shaft and the controls on the shaft itself, just below the engine. If the Stihl FS 55R starts to give you problems, troubleshoot it before calling a repair person.

Engine Not Starting

Mix 1 gallon of unleaded gasoline with 2.6 fluid ounces of Stihl 50:1 two-stroke engine oil in an approved fuel canister. Shake the mixture to thoroughly mix it up.

Clean the area around the fuel cap at the end of the trimmer. Turn the cap counterclockwise to remove it, fill the tank until you can see the gasoline level approach the opening, then replace the cap.

Pull up on the rubber boot over the spark plug, located on the very top of the FS-55 trimmer, to remove it. Rotate the spark plug counterclockwise to remove it from the trimmer.

Hold down both triggers, one on each side of the handle, until they lock into place. Pull the starter cord three times to clear the chamber.

Move the choke lever, on the left side of the machine, down to the lowest settings. The icon for the setting you want will look like a horizontal symbol between two horizontal lines. Press the fuel pump bulb, located right next to the choke lever, five times.

Clean off the spark plug, replace it and return the rubber boot. Pull the starter cord until the engine starts. Squeeze and release the throttle trigger to put the engine into idle mode.

Trimmer Has Poor Performance

Pull up on the rubber boot over the spark plug. Unscrew the spark plug and clean it off. Check the electrolode gap on the end of the spark plug, as this gap should be no larger than .5 mm. Push down slightly on the external electrolode to close the gap. Replace the spark plug and the rubber boot.

Move the choke lever to its highest setting, which has an icon that looks like a vertical line between two horizontal lines. Press in on the tab located just below the choke lever on the left side of the motor and swing the air filter cover open.

Clean any dirt from around the air filter. Pull out on the felt filter, knock it on the palm of your hand and blow on it to clean it. Do not get this filter wet. Replace the filter and close the air filter cover. If you have a replacement felt filter, you can install the new one instead.

Unscrew the filler hole cover on the side of the cutter head and check for grease. If you do not see any grease, screw a tube of Stihl gear lubricant into the filler hole on the cutter head. Squeeze 5 grams, which will only be a small amount, since most tubes hold 80 or 120 grams, into the head. Remove the tube and screw the hole cover back in place.

Mount the Stihl FS-55R trimmer so you can access the rear of the unit and the cutter head can spin without hitting anything.

Locate the three screws on the left side of the engine: two adjacent screws near the top and one screw near the bottom. The bottom screw is the idle speed control, the left top screw is the high speed control and the right top screw is the low speed control.

Use a Phillips screwdriver to turn the high speed screw counterclockwise as far as it can go, which will be a 3/4 turn at most. Turn the low speed screw counterclockwise as well, one full turn. Start the engine and let it warm up for about three minutes.

Rotate the idle screw clockwise until the cutting tool just starts to rotate, then turn it counterclockwise slightly to make the tool stop rotating. Make adjustments to the high and low speed screws to adjust the engine power when the throttle is down.