1978 Johnson 55 HP Outboards Service Manual, Page 18Get this manual
TROUBLE CHECK CHART (CONT) TROUBLE POSSIBLE CAUSE EOVERHEATING POWER HEADSee Section Thermostat cover or gaskets damaged Pressure control valve damaged Exhaust cover gaskets leaking Head gasket leaking (warped head) (water in cylinders) Water intakes obstructed (overheating) Pump housing air bleed restricted (overheating) Water passages obstructed (overheating) Pump plate damaged or warped Pump impeller damaged Pump housing distorted (overheated) Pump housing cover seal worn (driveshaft grooved) Water tube grommets damaged Water tube bent or not connected to pump FLOWER UNITSee Section Propeller hub Slipping Bent or worn propeller Bent gear housing or exhaust housing ASPARK PLUGSSee Section Cover or inner terminal damaged (terminal point out of H.Tlead) Faulty leads or crossed leads Loose spark plugs (low torque) Defective spark plugs (cracked insulator) Incorrect spark plugs Cylinder head spark plug threads damagedBIGNITIONSee Section Connectors improperly joined Coil leads loose Charge coil faulty Sensor coil faulty Sheared flywheel key Throttle cam improperly adjusted Loose armature plate screws or worn Delrin ring Power pack faulty