Mercury 35/40HP 2 Cylinder Outboards Service Manual PN 90-42794--1, Page 82Get this manual
Thermostat (if Equipped) Wash thermostat with clean waterUsing thermostat tester, similar to the one shown, test thermostat as follows: aOpen thermostat valve, then insert thread between valve and thermostat bodyAllow valve to close against threadbSuspend thermostat (from thread) and thermometer inside tester so that neither touches the containerBottom of thermometer must be even with bottom of thermostat to obtain correct temperature of thermostat openingcFi II thermostat tester with water to cover thermostatdPlug tester into electrical outleteObserve temperature at which thermostat begins to open(Thermostat will drop off thread, that was installed in Step "a," when it starts to open Thermostat must begin to open when temperature reaches 140-145F (60 -63C) Continue to heat water until thermostat is completely opengUnplug thermostat testerhReplace thermostat, if it fails to open at the specified temperature, or if it does not fully open