1978 Johnson 4HP outboards Service Manual, Page 57Get 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 rpm Using 163 (73 C) Thermomelt Stik, mark the side of the cylinder (preferably the core plug carrying the serial number) with the StikIf the Stik will not leave mark on the surface being checked, hold the Stik against the surfaceSee Figure 52
cIf starter does not require service, leave it attached to the crankcaseIf removal for service is necessary, see Section 7
DO NO use the stick on the cylinder headThe center of the cylinder head on the HP motor will normally run slightly hotter than 163 (73C) The mark will appear dull and chalkyIf the mark melts and appears glossy, the powerhead is overheatingCheck for worn water pump or clogged or leaky cooling system(See Section 6, Lower Unit)