1971 Johnson 2HP outboards Service Manual, Page 33Get this manual
CHECKING MOTOR TEMPERATURE
The motor may be checked for overheating with the use of the Markal Thermomelt StikThis is heat sensitive stick, similar to crayon, which melts on contact with surface at specific temperatureThe motor is best checked when operating on boatIf this is not poSSible, run the motor in test tank for at least five minutes, at maximum speed of 3000 rpmUsing 163 Thermomelt Stik, mark the starboard side of the cylinder with the StikSee Figure 5-2
REFERENCE PICTURE
DO NOT use the stick on the cylinder headThe center of the cylinder head on the HP motor will normally run slightly hotter than 163 FahrenheitFigure 5-2Checking Motor Temperature
The mark will appear dull and chalkyIf the mark melts and appears glossy, the power head is overheatingCheck for worn water pump or clogged or leaky cooling system(See Section 6, Lower Unit)