Johnson Evinrude Outboard Motors 1956-1970 1.5-40hp repair manual., Page 28Get this manual
TUNING Inspect the threads in the spark plug opening of the head and clean the threads before installing the plugIf the threads are damaged, the head should be removed and and Hell-coil insert installedIf an attempt is made to drill out the opening with the head in place, some of the filings may fall into the cylinder and cause damage to the cylinder wall during operationBecause the head is made of aluminum, the filings cannot be removed with magnetWhen purchasing new spark plugs, ALWAYS ask the marine dealer if there has been spark plug change for the engine being servicedCrank the engine through several revolutions to blowout any material which might have become dislodged during clean-
Damaged spark plugsNotice the broken electrode on the left plugThe broken part must be found and removed before returning the engine to service
between cylinders is far more important than the actual readingsA variation of more than psi between cylinders indicates the lower compression cylinder may be defectiveThe problem may be worn, broken, or sticking piston rings, scored pistons or worn cylindersThese problems may only be determined after the head has been removedRemoving the head on an outboard engine is not that big deal and may save many hours of frustration and the cost of purchasing unnecessary parts to correct faulty condition2-4 SPARK PLUG INSPECTION