My String Trimmer Starts, But Won't Stay Running

The two-cycle engine that runs your string trimmer needs three things to operate smoothly: clean fuel, air circulation and a spark. The engine won't start if any of these elements are missing, but it may run for a short time if only the air or fuel flow is restricted. It isn't difficult to restore air circulation, but if the fuel flow is restricted, you may need to disassemble the carburetor and clean it.

Air Circulation

The engine draws air into the combustion chamber through the intake port, and if the air filter is dirty, combustion is partial and the trimmer will sputter and die. The air filter is readily removable, and replacements are inexpensive. Similarly, air must exit through the exhaust port, and that is covered by a spark arrestor to prevent fires. When the spark arrestor is clogged, the engine can't get rid of its exhaust gases, and it won't keep running. The spark arrestor is a screen located just behind the muffler. Pull it out and clean or replace it.

Fuel Circulation -- Quick Fixes

The gas cap on most trimmer has a small air inlet hole that allows air to enter the fuel tank, preventing a vacuum from forming as fuel sprays into the carburetor. It's easy to check whether or not this hole is blocked -- just loosen the cap and see if the trimmer operation improves. If it does, it's time to replace the gas cap. The fuel system also includes a filter that can clog. It's difficult to determine visually whether or not it's blocked, but it's easy and inexpensive to replace. You should do this as part of regular maintenance.

Old Fuel

If you leave the trimmer full of gas when you store it for the winter, the fuel partially evaporates, leaving behind a sludge that resembles varnish. This sludge can partially block the inlet ports for the carburetor. Consequently, there may be enough fuel in the carburetor to start the engine, but there may not be enough to keep it running. You can prevent this from happening by adding fuel stabilizer to the gas tank before you store the trimmer, but once the carburetor is blocked, there's no alternative to cleaning or replacing it.

Servicing the Carburetor

While it can be a delicate task to disassemble a carburetor for a complete cleaning, you usually don't have to do that to remove deposits from old fuel. The first step, after you've located the carburetor, is to drain the float bowl. After unscrewing and removing it from the trimmer, spray it generously with carburetor cleaner, being sure to direct the cleaner through all the port orifices. If the trimmer doesn't run smoothly after doing this, disassemble for a more thorough cleaning. It's probably best to take the carburetor to a service professional for this.

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