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Part of the Mariner line of outboard motors, the four-cylinder two-stroke 115-horsepower Mariner even shares the "Optimax" name with the Mercury motor of the same horsepower rating for a simple reason -- both are built by Mercury Marine. Mariner Outboards came into being in 1977 as the "export" version of Mercury Marine's outboards.
Size, Oil and Fuel Capacities
Weighing in at 347 lbs, the engine's displacement was 93 cubic inches, or 1,848 cc, with a cylinder bore of 3.50 inches and a piston stroke of 2.93 inches. An oil-injected motor, it didn't require you to pre-mix the fuel and oil together; rather, 5.3 quarts of oil filled the precision oil injections tank, to be mixed in a 50-to-1 ratio as required by the motor. Because the fuel tank was a "remote" tank rather than a tank incorporated into the motor's structure, the capacity was user-controlled. The lower unit, where the gears and propeller shaft are housed, requires 22.5 ounces of lower unit lubricating oil.
Idle and Maximum Speeds
The Mercury 115's idle speed ranges between 625 and 675 rpm. The idle speed is not adjustable and is controlled, as is the timing, by the electronic control unit. The maximum speed of the motor, however, isn't controlled by the ECU. The motor's maximum speed -- its redline, where it begins to beg for mercy and the motor speed you should not exceed -- is 5,000 rpm. It hemorrhages internally and suffers a catastrophic failure at 5,750 rpm.
The Mercury 115's ignition timing is pre-set at the factory and controlled by the Electronic Control Unit. While this make it impossible for the weekend boater to adjust the timing, should the motor regularly misfire or operate roughly, you may test the capacitor charging coil, trigger and ignition coil with a multi-meter, but there is no guaranty that those are the only potential trouble spots. If the Mercury 115 is running rough, it needs a dealer's care.
The Cooling System
The cooling system on the Mercury 115 is the same as most outboards; raw water is sucked into the motor from the body of water in which the motor operates, pumped throughout the powerhead's cooling galleries. The system's operating temperature is regulated by a thermostat and pressure-release valve. The thermostat and the pressure-release valve are located under a small irregularly shaped cover on the upper end of the cylinder head, held in place by four bolts.
- "Mariner/Mercury Outboards Repair Manual, 2.5-250 HP, 1990-2000"; Seloc Marine; 2007
Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.