1981 Johnson/Evinrude 4HP Outboards Service Manual, Page 44Get 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 minutesat 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 Stik DO NOT use the stick on the cylinder head or exhaust coverThe center of the cylinder head on the hp motor will normally run slightly hotter than 163 (73 C)If the Stik will not leave mark on the surface being checkedhold the Stik against the surfaceSee Figure 5-2The 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)