DIAGNOSIS Excessive vibration indicates an imbalance in the rotating assembly. The most likely causes are a bent crankshaft, a failed anti-vibration mount, or a severely unbalanced cutting attachment.
FIX First, eliminate the bar and chain as sources. Remove them and run the engine briefly at idle. If vibration is gone, the problem is there. Inspect the bar for straightness and wear. Ensure the chain is properly tensioned and the drive links are not unevenly worn. If vibration persists without the bar, check the four anti-vibration mounts where the engine connects to the rear handle. Replace any that are cracked or squashed.
IF THAT DIDN'T WORK The internal crankshaft or engine bearings are damaged. This is a major repair. Signs include a visible wobble in the clutch drum when the engine runs or metallic grinding noises. This requires complete engine disassembly.
HEADS UP Continued use with severe vibration will damage every component in the saw and is a significant safety risk. Stop using it until the source is found and fixed.