One likely reason your weed eater won't start is because the weed eater pull cord is broken. In some cases, it may need to be replaced; if so, the procedure depends on the brand and model. If you have your weed eater's manual, that may be useful.
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A Broken Weed Eater Pull Cord
Some of the best known weed eater brands are Poulan, Stihl, Ryobi, Black & Decker and Craftsman. Oftentimes, their pull cords don't work because they have snapped. Garden Tools & Instruments explains that if you need to do a Poulan Pro weed eater pull string replacement, first make sure that the tool is turned off and cooled down.
Loosen the screws that hold the starter cord cover. Pull this off and remove the cord, which should be attached. Next, pull out the whole starter cord from its pulley. Now, open the plastic starter cover with the screwdriver and untie the rope that's inside the handle and discard it. Put your new cord into the access hole in which the old cord was attached, and push it through and lock the center tab down onto it.
Completing the Assembly
You can now thread this new cord into the metal insert and the handle. Tie a loose knot on the end and push this knot inside the handle. Tighten the knot next to the insert and close the handle over the knot.
The cord should now be wound around the pulley; holding the cover in one hand, pull the cord around in a few rotations to provide some tension on the recoil spring. Replace the cover, and you're done.
Other Kinds of Weed Trimmers
If you have a Craftsman weed eater, Garden Guides shares the steps for replacing this kind of pull cord, which is not all that different from the Poulan. Turn the weed eater off and remove the starter assembly cover with a screwdriver. Cut off the old string from the pulley and starter handle and discard it. Take the new cord, push it through the pulley and thread it through the starter cover hole and starter handle.
Tie your knot above the trimmer's starter handle at the pulley base and wind it around the pulley, leaving 10 inches, then make a loop. Pinch the bottom and work it back and forth until recoil spring hooks itself onto your pulley. Holding the starter cover, wind the pulley and loop around several times until there is enough tension. Hold the pulley, release the loop and unwind the new cord. Slowly let it rewind onto the pulley, ensuring that it is tight, and replace the starter cover.
General Safety Tips
Whenever working on power tools, be sure to turn them off first and to allow them to cool down completely. Wear a pair of gloves to protect your hands.
If you need to order new parts like replacement pull cords, write down the model number and go to the manufacturer's website. You may be able to order the parts directly from the manufacturer or an online retailer.