2003-2006 Kawasaki KFX400 service manual, Page 104Get this manual
VALVE SPRING The force of the coil spring keeps the valve seat tightA weakened spring results in reduced engine power output and accounts for the chattering noise coming from the valve mechanismCheck the valve springs for proper strength by measuring their free length and also by the force required to compress themIf the spring length is less than the service limit or if the force required to compress the valve spring is not within specification, replace both the inner and outer springs as setValve spring free length (INEX) Service Limit: 38.8 mm (1.53 in) Valve spring tension (INEX) Standard: 182 210 (18.6 21.4 kgf, 41.0 47.2 lbs) at length 31.5 mm (1.24 in)