The force of the coil spring keeps the valve seat tight. Weakened spring results in reduced engine power output, and often account for the chattering noise coming from the valve mechanism.
Check the valve springs for proper strength by measuring their free length and also by the force required to compress them. If the spring length is less than the service limit, or if the force required to compress the sprizng does not fall within the range specified, replace it.
09900-20102: Vernier calipers
Valve spring free length (IN & EX)
(20.1-23.1 kgf/35.6 mm, 44.3-51.0 lbs/1.40 in)