I have a John Deere (Echo?) XT140SB split boom trimmer that decided not to start this year. When I try to start it, I get a jerking motion from the starter cord instead of a smooth pull. I pulled the spark plug out, and the starter cord pulls just fine like it's designed to do. Put the spark plug back in and the jerking motion comes back when trying to start it. I pulled the carb loose and the same jerking motion happens since the plug is still in the head. The piston moves properly so I know it's not a broken connecting rod.
Since it appears to be getting spark and the carb seems to be getting fuel; I'm concluding that there is something wrong in the cylinder/head assembly. A stuck or broken reed valve maybe? According to the JD Parts web site the cylinder/head assembly is replace only.
The trimmer is 9-years old.