Poulan weed eaters are gas-powered two-cycle engines, and like all of these small engines need only three basic things to start and run: spark, compression and fuel. Even though they're small engines, they still have a variety of parts that can malfunction, causing a wide array of problems. To solve these problems, it is best to isolate the area of the problem into one of the three main components. This will allow you to find quickly the specific parts that need repairing, instead of ripping the whole thing apart.
Take off the air filter cover using the screwdriver; pull out the air filter and slap it against your palm, shaking off any dust or debris. Replace the filter if it's old or too dirty to clean. Use a little gas, a toothbrush and a rag to clean the areas around the air filter and cover on the trimmer.
Use the screwdriver to take off the muffler's cover; slide out the spark arrestor screen. If possible, clean this with the toothbrush and rag or replace it if it's heavily covered in black carbon build-up. Check the exhaust port inside the muffler for any clogs or damage.
Pull the trimmer up off the ground by the starter cord. If the engine sags under its own weight, pulling the cord with it, then your compression is low. Take off the starter cover and inspect the starter cord, pulley and recoil spring for any damage; replace if necessary.
Retune your carburetor (if you can start it) and perform a compression test (take it to a mechanic if you don't know how). You may have a problem with your reed valve, piston seals or somewhere in the crankcase if your compression is still low. The trimmer should still run with low compression, but the engine will be sluggish and poor.
Ignition and Spark Problems
Unscrew the spark plug and remove it from the engine using the socket wrench. Remove all of the built-up deposits with a little gas, a toothbrush and a rag, if you can. Refer to your owner's manual for your model's gap recommendation and make sure it is properly gapped.
Check the wire and rubber boot plug for any damage or loose connections. Inspect the spark plug hole for any debris or damage inside the cylinder housing.
Put the spark plug into the rubber boot plug, but don't attach the spark plug and the rubber boot to the engine yet. Let the spark plug and rubber boot hang beside a metal point on the engine. Tug lightly on the starter cord and look for a spark across the metal and the spark plug. Change the spark plug and repeat test if there's no spark.
Test the ignition coil, starter and other ignition wires for any damage that may be causing your loss of spark.
Fuel System Problems
Clear the gas tank of all old gas. Spray a little starter fluid onto the rag, wrap it around the toothbrush and scrub the inside of the tank. Look for any damages or warping inside the tank.
Disconnect the fuel line from the carburetor and remove the old fuel filter and fuel line from the trimmer. Check the seals around the carburetor and gas tank line for any leaks or damage. Change out the fuel filter and fuel line for new parts. Put the trimmer back together and try starting it again. If it starts, adjust the carburetor.
Take out the carburetor and disassemble it, if the trimmer still won't start. Let all of the pieces of the carburetor soak in carburetor cleaner for at least one full day. Scrub them clean with a brush and replace any worn seals, gaskets or parts too dirty to be cleaned with the carburetor kit.