How Does A String Trimmer Work?

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As an essential landscaping power tool, a string trimmer puts the finishing touches on a well-maintained yard by cutting back grass and weeds around trees, edging, and other areas difficult to reach with a lawn mower. Understanding how a string trimmer actually works will help you to diagnose any problems you may be having with the equipment and which string trimmer parts may need to be replaced to return the trimmer to prime operating condition.

It all starts with the engine

While string trimmers can be powered by a battery, a power cord, and even propane, the most common trimmers are gasoline-powered. Commercial gas string trimmers are often powered by a four-cycle engine, whereas most residential string trimmers use a two-cycle engine which normally requires the gasoline to be mixed with oil for the engine to have sufficient lubrication to operate.

To start the engine, the ignition switch must be set to the start position. As the starter rope is pulled, the starter engages the drive cup on the flywheel and rotates the crankshaft. The rotating crankshaft connects to a piston which moves up and down within the cylinder. As the piston travels downward, an intake port is exposed where fuel, oil, and air enter the cylinder. As the piston travels back up the cylinder, a vacuum is created and the gas, oil, and air is drawn through the carburetor into the crankcase. The flywheel has permanent magnets built into it, and as it moves past the ignition coil a magnetic field is created which induces electricity, allowing the ignition coil to send a high-voltage pulse to the spark plug. When the piston reaches the top of the cylinder, the spark plug ignites the compressed fuel and air mixture, forcing the piston back down. The spent fuel exits an exhaust port, fresh fuel enters the intake port, and the combustion process is repeated to keep the engine running.

String trimmer engines will have a choke, primer bulb, or both, to assist with starting the engine, especially when the engine is cold. The primer bulb will draw additional fuel through the carburetor, and the choke will temporarily restrict air flow through the carburetor so more fuel can enter the cylinder. The exhaust from the engine travels through a muffler, which reduces engine noise, and a spark arrestor which is designed to prevent burning carbon deposits from exiting the engine housing and potentially starting a fire.

String trimmer engine trouble

You can prevent problems from developing with the string trimmer engine by performing an engine tune-up once a year. This normally involves replacing the spark plug and air filter, but you should also consider replacing the fuel filter and using a fuel stabilizer to maintain the quality of the fuel. You may also need to purchase a carburetor repair kit to replace some of the carburetor components like the float, needle, gaskets, and diaphragms. Here are some additional troubleshooting tips regarding the engine:

  • If the engine starts, but stalls shortly after, it’s likely that the spark arrestor is clogged with soot or the carburetor is partially restricted.
  • If the engine doesn’t start at all, the spark plug may be defective or the carburetor may be fully restricted.
  • A restricted carburetor is caused by the residue of old fuel being left in the tank. You may be able to clear the restriction by using WD-40 or a carburetor cleaner to clean out the carburetor ports.
  • To help prevent carburetor restriction and improve fuel stability, use a pre-mixed fuel and oil product that is ethanol-free instead of mixing your own fuel.

What makes the trimmer head spin?

The crankshaft of the engine extends out the side of the crankcase and connects to a centrifugal clutch which engages with the drive shaft attached to the trimmer head. When the engine runs at idle speed, the clutch prevents the drive shaft from rotating. When the trigger is pulled, a cable opens the throttle on the carburetor which accelerates the engine. This causes the clutch flyweights to extend outward and engage the drive shaft which rotates the trimmer head.

As the trimmer head rotates, the trimmer line spreads out to cut through grass, weeds, or low foliage. The size and thickness of trimmer line varies depending on the trimmer model, but the most common diameters for residential use range from 65/100th of an inch to 95/100th of an inch. The trimmer line may be wound around a spool, or the line may be cut into small individual strips that are loaded into the head. Most wound trimmer heads have a bumper, or bump knob, that can be tapped on the ground to feed additional line out of the head as the line is worn down, although newer models have an “auto-feed” feature. If too much line extends out of the trimmer head, a blade located on the guard will trim the line as the head rotates. If the trimmer runs out of line, the head can be re-spooled or the entire head can be replaced.

Common string trimmer problems

  • If the trimmer head does not rotate when the engine is accelerated, the clutch may have failed or the drive shaft may have stripped out.
  • If the engine fails to accelerate, the throttle cable may have broken.
  • If the trimmer line won’t feed, the trimmer head could be obstructed or worn out, the trimmer head housing could be damaged, or you may be using the wrong-size trimmer line for your model.

Find the right string trimmer parts with Repair Clinic

To find the right tune-up kit for your string trimmer engine, as well as replacement parts for both the engine and the string trimmer itself, enter the full model number of your product in the Repair Clinic website search bar. You can then use the part category and part title filters to narrow the search down to the exact part you need, whether it’s a new carburetor, throttle cable, trimmer head, or trimmer line. Repair Clinic carries string trimmer parts for all the top brands, including Craftsman, Echo, Husqvarna, Kawasaki, MTD, Poulan Pro, Ryobi, Toro, and Troy-bilt, but you’ll want to make sure you’re purchasing the specific part that fits your string trimmer model.

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