I posted this thread a few days back.
Yanmar De-Compression Levers
On my 3GMD the decompression levers have no adjustment. There is a detent ball that keeps the lever in place. The detent ball is just a small metal ball bearing held against an indentation in the decompression lever rod shaft that holds it in place. The spring or indentation in the rod may be worn and does not hold they lever hard enough thus it "pops" out. When in decompress the lever does not push the exhaust
valve down as far as the push rod- thus the vibration. If the decompression lever held the exhaust
valve down its full stroke, the piston would strike it at its Top Dead Center and do piston/valve damage.