1977 Johnson 2HP Outboards Service Manual, Page 37Get this manual
5-3 MOTOR
The motor checked for overheating with the use of the Markal Thermomelt StikThis is heat sensitive stick, similar to crayon, which melts contact with surface at specific temperatureThe motor is best checked when operating boatIf this is not possible, run the motor in test tank for at least five minutes, at maximum speed of 3000 rpmUsing 1630 Fahrenheit (72 Celsius) Thermomelt Stik, mark the starboard side of the cylinder with the StikSee Figure 5-2
REFERENCE PICTURE
DO NOT use the stick the cylinder headThe center of the cylinder head will normally run slightly hotter than 1630 Fahrenheit (72 elsius)The mark will appear dull and cha1kyIf the mark melts and appears glossy, the power head is overheatingCheck for worn water or clogged or leaky cooling system(See Section 6, Lower Unit)REMOVAL OF POWER HEAD