Lisandro Guzman, The spark arrestor is a small screen that prevents the engine from emitting sparks. Over time, the spark arrestor can become clogged with soot/oil build up. If the spark arrestor is clogged, the engine may stall. Remove mufler to access arrestor screen. To unclog the spark arrestor, remove it heat screen with propane torch and clean off ash/residue with a wire brush. You may also replace the spark arrestor. Also look for build up of carbon in the engines exhaust port and carefully scrape port clean(do not let carbon fall in cylinder or could cause damage). Other possibility is fuel cap vent plugged, remove gas cap (careful of fuel spillage) and see if the engine runs better. Carberator out of adjustment and sometimes these small engine carbs. still will not operate after rebuild. Check links below to see how to clean /check spark arraestor screen. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.
https://www.youtube.com/watch?v=U0LJ3gX2_Cw
http://www.ereplacementparts.com/article/17443/How_to_Replace_the_Spark_Arrestor_Screen_on_an_Echo_Trimmer_(Model_SRM225).html
https://www.repairclinic.com/vwo14/RepairHelp/How-To-Fix-A-String-Trimmer/156-349-1136070-/Echo-String-trimmer-starts-then-stalls-SRM2100SERIAL289258512721