- Loosen the two screws that hold the air filter cover in place. Lift up the cover and take out the air filter pad. Brush off the air filter cover, the pad and the area around the pad, including the cylinder's fins. Drain any leftover fuel that's more than 72 hours' old from the fuel tank. Dispose of it according to local regulations. Mix fresh fuel for the ST-275 and fill the fuel tank. Never adjust the carburetor with bad, old or less than ¾-tank of fuel.
- Set the trimmer on the ground so the head isn't touching anything as it will rotate during carburetor adjustment. Locate the carburetor adjusting screws in between the air filter box and the power head on the side of the carburetor. Start the engine of your ST-275 and let it run at a high idle for five minutes to warm up. Insert a screwdriver into the screw marked "Idle." Some models may have only one screw: this is the idle and low speed screw. Other models have two: one for the idle speed and one for the high speed.
- Adjust the screw clockwise until the trimmer head starts rotating. Stop moving the screw as soon as it starts spinning and turn it counterclockwise until the head stops moving. Move the screw slowly clockwise again and stop just before the trimmer head starts to spin. The ideal idle speed is the maximum speed before the engine starts to turn the cutting head. Never operate the ST-275 trimmer if the head continues to spin while idling.
- Over-adjusting the carburetor can cause serious damage to the carburetor and the engine. Leave the plastic limiter caps in place to prevent over-adjusting. Removing the plastic limiter caps for carburetor adjustment may void the ST-275's manufacturer's warranty. Don't attempt to adjust the low speed or high speed screws without professional help. Never adjust the carburetor on the ST-275 with a dirty air filter or before the engine has had time to warm up.
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