How to Tune Up a Homelite String Trimmer ST-145

Written by eric blankenburg
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How to Tune Up a Homelite String Trimmer ST-145
Adjust your string trimmer carburettor to ensure optimal engine results. (line trimmers image by Horticulture from

The Homelite ST-145 comes properly tuned from the factory, but over time the carburettor needs to be adjusted to keep the air fuel mixture exact. If this mixture is off, you will likely notice problems during acceleration and when the trimmer is idling. You may also have difficulty starting the machine. Tuning the carburettor is a simple process which you should perform on a consistent basis to keep the machine in peak performance. Developing a good ear for the proper engine sound is key to keeping your carburettor properly tuned.

Skill level:

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Things you need

  • Fresh fuel mixture
  • Approved gas container, empty
  • Screwdriver
  • Small screwdriver
  • Soapy water
  • Wire brush

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    Before Tuning the Carburetor

  1. 1

    Dump out any old fuel still in the tank into a container approved for gasoline. Make sure the fuel filter inside the tank is not clogged or dirty. Replace the fuel filter if necessary. Fill the trimmer with fresh gas at a fuel-to-oil mixture of 40:1.

  2. 2

    Examine the check valve on the gas cap to ensure it is open and not clogged. Replace the gas cap if necessary.

  3. 3

    Depress the tab on the air filter cover to remove it. Take out the air filter, and wash it in warm soapy water until it's clean. Let it dry thoroughly before putting it back in. Replace the air filter if it's too dirty to clean.

  4. 4

    Unscrew the muffler cover, and remove the spark arrester screen. Clean off the black carbon deposits on the spark arrester screen with the brush and warm soapy water. Look inside the muffler and exhaust port for any clogs or heavy carbon build-up. Replace if they're heavily clogged or broken.

    Tuning the Carburetor

  1. 1

    Start the trimmer, and let it warm up for a few minutes. Depress the throttle a few times to clear the carburettor of any old gas.

  2. 2

    Locate the idle speed screw that's next to the "A-B-C" choke lever and is behind the air filter.

  3. 3

    Turn this screw all the way in clockwise until the engine makes a high-pitched whiny noise. Turn the screw back out 1/8 of a turn. Remember this position of the screw.

  4. 4

    Turn the screw out counterclockwise until the engine produces a heavy bubbling sound. Turn the screw back in 1/8 of a turn; remember the position of the screw.

  5. 5

    Fine-tune the carburettor in between the two settings found earlier. Listen carefully to the engine. For best results, it should have a clean, smooth sound.

Tips and warnings

  • Never try to tune the carburettor on a cold engine.

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