2000 Yamaha YFM400FWA(M) Factory Service workshop Manual, Page 235Get this manual
TROUBLESHOOTING CAUTION:
Too little gear lash is extremely destructive to the gear teethIf test ride following reassembly indicates this condition, stop riding immediately to minimize gear damagec.A slight "thunk" evident at low speed operationThis noise must be distinguished from normal machine operationDiagnosis: Possible broken gear teeth
WARNING