2003 Johnson ST 6/8 HP 4 Stroke Outboards Service Manual, PN 5005471, Page 115Get this manual
POWERHEAD ENGINE TEMPERATURE CHECK
ENGINE TEMPERATURE CHECK
Temperature Gun, PIN 772018, or digital pyrometer should be used to accurately determine the engine's operating temperatures
Temperature Gun or Digital Pyrometer Test Procedure
Install correct test propeller and place outboard in test tank&
WARNING
Avoid accidental starting of engine while servicing; twist and remove all spark plug leadsStart outboard and run at 3000 RPM for at least five minutesDigital Pyrometer
To achieve accurate and consistent readings, engine temperature must be gauged at the top of cylinder head
Reduce engine speed to 900 RPMAt 900 RPM, the digital pyrometer or Temperature Gun should indicate 140 15F (60 rC) when checking at top of the cylinder headIf engine temperature is not within this range, troubleshoot cooling system to determine problem