DIAGNOSIS This is unbalanced reciprocation. The primary cause is a worn or damaged counterweight. The counterweight balances the piston. If it's damaged, vibration increases significantly.
FIX Disassemble the tool to inspect the counterweight in the gear case. Look for cracks, galling, or excessive wear on the counterweight surfaces. A damaged counterweight (part of assembly 02-06-2820) must be replaced.
IF THAT DIDN'T WORK The secondary cause is a bent reciprocating shaft (the part the blade attaches to). This is less common but can occur from extreme misuse or a major jam. Inspect the shaft for straightness.
HEADS UP Excessive vibration is not just uncomfortable. It indicates internal mechanical failure that will worsen. Continued use can destroy the gear case.