2000 Yamaha YFM400FWA(M) Factory Service workshop Manual, Page 145Get this manual
CAMSHAFT, ROCKER ARMS AND VALVES
10.Measure: Valve spring free length Out of specification ReplaceFree length (valve spring): Inner: 39.9 mm (1.57 in) Limit: 37.9 mm (1.49 in) Outer: 43.27 mm (1.71 in) Limit: 41.27 mm (1.62 in) 11.Measure: Compressed spring force Out of specification ReplaceCompressed spring force: Inner: 104.9120.6 at 33.6 mm (10.7012.30 kg, 23.5827.11 lb at 1.32 in) Outer: 235.4251.1 at 36.6 mm (24.0025.60 kg, 52.9256.45 lb at 1.44 in)