DIAGNOSIS The cutting blade is dull or damaged. The MJ401E uses a single 14-inch steel blade that dulls quickly, especially near fences or on sandy soil.
FIX Unplug the mower. Tip it on its side with the air intake vents facing up. Remove the blade bolt (requires a wrench). Inspect the blade for nicks, bends, or a worn edge. Sharpen it with a file to restore the original cutting angle, or replace it with part number MJ401E-BL. Reinstall, ensuring the blade is seated correctly on the adapter.
IF THAT DIDN'T WORK Check the blade adapter. If the blade spins but wobbles visibly, the adapter may be worn or the motor shaft is damaged. This requires part replacement.
HEADS UP Balance the blade after sharpening. An unbalanced blade causes excessive vibration and motor strain. Use a simple blade balancer.