America's Definitive Marine Engine Magazine
1. Keep the gasoline fresh and clean and dry (no water in gas). Sometimes hard starting is due to moisture in the fuel tank. To remedy, simply dose the fuel with a can of special additives that absorbs the water. Another method, the one I prefer is to loosen the drain screw in the carburetor fuel bowl. Opened, whatever liquid is in the fuel bowl will flow out. Capture it if you can. Look for bubbles, a telltale sign of water contamination. Water and oil don't mix, which is why you see the bubbles. Know that healthy fuel is clear and water-contaminated fuel is cloudy and murky.
2. Routinely check the propeller for fishline wrapped around the shaft. If you don't remove it, it burrows through the propeller shaft seal and cuts it. When the engine is running, even in neutral, the spinning gears pressurize the oil blowing it out through the cut in the seal. Then when the engine stops, the vacuum sucks in water to replace the lost oil. According to Pennzoil engineers, as little as an ounce of water can destroy a gearset.
3. On a four-stroke outboards change the oil and oil filter after its first 20-hours then every 100-hours thereafter, or at least once at the end of the season. Scrupulous attention to oil changes is neccessary because four-strokes are particularly sensitive to dirty oil. Dirty oil destroys crankshaft and connecting rod bearings. Because of their minimal oil capacity, four-stroke outboards are especially sensitive to diry oil. Carbon is abrasive and wears crankshaft bearings, piston rings and skirts and valve guides.
4. Change gear case oil on schedule. Once a year keeps fresh oil lubricating gears and bearings, extending life.
5. Stabilize fuel so it doesn't gum up the fuel system during dormant periods. Gasoline begins to sour after about a month. Special additives keep it fresh.
6. With saltwater motors, flush the engine with freshwater after operation. This clears salt crystals and other harmful debris from the cooling system. On some engine you need an inexpensive adaptor that fits the common garden hose. Know that at about 180 degrees salt precipitates out of solution and builds up on the internal cooling passages. A freshwater rinse washes them away.
7. Fog the cylinders if the engine will remain dormant for more than a couple of weeks. Fogging oil is sticky and vaporous and protects against rust.
8. Bolt the motor to the transom. Don't rely on just the clamp screws. Clamp screws are notorious for unwinding and letting the engine hope off into the water. Many outboards include a bolt for permanently locking them to the transom.
9. Water pump impellers do not age well. The rubber hardens, the impeller blades take a rigid set and stop pushing water. Sometimes they break off and flow upstream into the engine, sometimes clogging a water passage. Consider changing the impeller every two years as a preventive measure.
10. Finally, invest in two things: A. From West Marine or Raccor an appropriately-sized fuel filter funnel to remove particulate matter from the gasoline and at the same time remove any water contamination that may be lurking; B. Plumb a water-separating fuel filter between the fuel tank and the engine. The best ones are 10-micron, either from Yamaha Marine or Racor.