I went with brand new plugs and tried several, including taking one from a working trimmer and trying it in the problem trimmer.I'll assume a few things here....regap your plug, .030 plus a blonde one so it's a little wider.
Check the mag/flywheel that the key is not sheared/missing. I don't know if Echo has the molded key on the flywheel or if they use an woodruff key. But it sounds like your timing is off. Inspect the key if it is molded for any wear.
Lastly, next time you try to start it, take the muffler off. A carboned up/bad cat muffler would /could make to much back pressure.....
That was my plan. I just wanted any other input someone might have before ripping this down for the third time.You went from a working machine to this...I would have to assume that the short block is defective at this point. Reach out to Echo and see what they say..
ECHO is a manufacturer of professional-grade, dependable handheld outdoor power equipment - high quality products like chainsaws, trimmers, blowers and other power tools.www.echo-usa.com