A gas-powered string trimmer burns fuel to create power, which also creates an enormous amount of heat. Through several engine processes during normal operation, the engine heat gets vented away from the block and into the outside air. If the trimmer can't vent gases, it will quickly overheat, and that can affect the trimmer's performance and damage the engine.
It's possible the overheating is caused by a defect in the engine, but most of the time, the problem is a blockage in the air passages or a carburetor that's running too lean. These are problems you can fix yourself — no need to bring the trimmer in for servicing unless the problem does turn out to be an engine defect.
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Clean the Air Passages
The air passages on a gas powered string trimmer regulate the breathing function of the engine. The air filter helps bring clean, cool air into the engine, and the muffler helps push heated gases away from the engine. If either of these passages becomes blocked or dirty, the engine won't regulate the incoming and outgoing air, and it will overheat. Remove the pad part of the air filter, and wash it in soapy water. Rinse under cool water, and let it dry thoroughly. Clean out the muffler and spark arrestor screen with a wire brush.
Clean the Cooling System
The cooling system on a string trimmer engine also helps heat escape. Heat radiates outward, and cool air gets pulled in through the flywheel. Most gas powered string trimmer cooling systems are accessed through drive shaft removal. The cylinder cover on top of the engine also acts as part of the air-cooling system. All of these areas, including the starter system and flywheel, need regular cleaning to prevent trapped particles from blocking the air supply.
Check the Fuel Level
A gas powered string trimmer operates on a two-cycle engine, which requires a pre-mixed fuel. This fuel includes the engine oil to keep the crankcase and piston adequately lubricated. If this mixture is off and there's too much gas in the mixture, the fuel will burn at a higher temperature, causing the engine to overheat. Mixed fuel can also separate and go bad after 30 days. As the oil separates from the gas, it will float to the bottom of the mix and the mixture will be too lean. Follow a strict 50-to-1 gas-to-oil ratio, unless otherwise indicated by the instructions in your product manual.
Readjust the Carburetor Screws
After cleaning the air and cooling systems and checking the fuel supply, it may be time to readjust the carburetor, especially if the engine is still overheating. As an example, the carburetor on a Weed Eater brand lawn trimmer uses three adjustable screws to regulate the amount of gas at a given engine speed. The same is true for most other brands. If too much gas is being used, the engine will speed up and run hot. Most trimmer manufacturers require that carburetor adjustments be performed by a service professional, but any amateur gardener with a sense of how small engines work can do it.