OMC Stern Drives And Motors 1964-1986 Repair Manual., Page 42Get this manual
TROUBLESHOOTING t spark plugs shorted one-at-at until the engine is running on one cylinderNow, if vacuum gauge is attache the intake manifold, reading can be obtained compare the efficiency of cylinderA low-reading cylinder can the result of inefficiency in the ignition, fuel, compression systemT make this test the ground clip of the shorting wires must attached good groundS the engineCAUTION: Water must circulate through the stern drive the engine any time the engine is run prevent damage to the water pump in thc stern driveHave helper advance the throttle as you successive1y short out a11 of the cylinders until the engine is running on one cylinderYou will discover will be necessary open the throttle wide in order keep the engine running on cylinder at timeObserve the vacuum gauge reading, and then move the shorting clip from one of the spark plugs the spark plug of the cylinder you just testedObserve the vacuum gauge reading, and then test of the other cylinders in turn by running the engine on one spark plugA weak cylinder or cylinder is not firing is easily determined, but the most important part of this is the ability compare the relative power (vacuum) of firing stroke