Over the winter, or for any period exceeding three months, you will need to put your string trimmer into storage mode. You can't just set it on a shelf, forget about it four five months and expect it to run like new when spring arrives. Old gas will separate and dry, leaving behind a sticky substance gumming up your fuel system.
Getting Rid of the Fuel
If you don't plan on finishing the fuel left inside the tank, you must dump the remaining fuel into an approved fuel container. This fuel will need to be disposed of according to local environmental regulations, so do not dump it out into the yard or on the street. With an empty tank, start the string trimmer up, and let it run at a high idle until the machine dies, which will ensure all fuel in the carburetor has burnt off, preventing the diaphragms in the carburetor and fuel pump from sticking together. It will also ensure the jets and bores remain clear and open.
Cleaning the Tank
After you've drained the tank, burned off any fuel in the lines and carburetor, you will need to give the fuel tank a thorough cleaning. Leftover gas deposits will stick to the walls, and when you refill the tank next summer, those sticky deposits may come loose and end up in your carburetor. You can clean the tank with a firm, metal brush and a rag. If you need a little gas to get out any big chunks of dirt or grass, pour a little into the tank and swirl it around. Whether you do it now or in the spring, you'll need to put a new fuel filter onto the end of the main fuel line. Using a new fuel filter will ensure only the cleanest gas is reaching the carburetor.
Cleaning the Machine
After cleaning out the tank, you'll want to give the entire machine a good cleaning. You can use the same brush and rag to wipe off any grass clippings. Pay close attention to the air vents around the muffler, the fins on the cylinder and the air filter cover. You'll also need to remove and clean the air filter. Don't forget about the shaft and the cutting head. If you have access to a power washer, give them both a good spraying; just try not to get any water into the engine's electrical system.
Storing the Machine
You'll want to choose a place in your garage or tool shed that's high enough to keep it out of reach of children and other unwanted users. If you have a locked area, this will be best to ensure no one but you uses the machine. It will also need a dry place as damp moisture can corrode the internal engine parts.