Last Gasp. Rev your engine before turning it off. It was probably a mechanic needing work who started this myth. The idea is that it's supposed to make the engine easier to restart. But all it does is leave unburned fuel coating the cylinders, which forms a gummy deposit.
Flick the Switch. Turn the battery selector switch incorrectly while the engine is running and, bam! No more alternator. When switching from 1 to 2 or Both, never pass through the Off position. For the microsecond the switch is completely closed, the alternator's output has nowhere to go. This will blow out the diode and kill the alternator. Prevent this with a $13 gadget called Zap-Stop. It automatically directs any charge over 16 volts to ground.
Anode Ignorance. Sacrificial zinc alloy blocks go only on your drive or propshaft, right? Wrong. Your engine has an internal one, too. You'd better find out where it is and learn to replace it every time you change your oil. If left in place too long, it will start to crumble and give off particles that may clog your cooling system.
Stainless Steal. Assume that if the ring of a hose clamp is stainless, the spiral screw that tightens is made of the same stuff. Often the screw is mild steel, which rusts if you put a salt shaker near it. A broken hose clamp is a sure way to lose water, fuel, or an exhaust hose-or to sink a boat.
Dangle a Line. Leave lines hanging off the stern. Wrap some 3?8" nylon around your prop at a few thousand rpm and bearings get shot, drip glands rip up, transmission gears strip, and if the engine mounts are weak, the engine pulls out of alignment or loosens from its bed.
Neglect Corrosion. Ignore your engine's need for a dry environment. Keep electrical connectors coated with WD-40 or Vaseline to repel moisture. If boating in saltwater, occasionally wipe down the engine with a rag damp from freshwater. Make sure you get rid of all dried-on salt crystals, which attract moisture. Wipe dry, spray on a light coating of oil, and spread that oil around to make sure all parts (even undersides) get protection.
Don't Change. Don't change your oil-just dilute what's been left behind. Your goal is to completely replace the old oil with new. Compare how much oil comes out with what goes in. The difference is the amount of foul oil left behind. If you can't suck out all the old oil from the top, install an oil draining kit under the sump. Just make sure the engine's mounting angle allows oil to collect over the drain plug. Or, change your oil more frequently so that the oil left behind never has a chance to get bad.
With This Ring. Purchase the cheapest fuel-'round here we call it "frog wizz." You will not only get the worst possible performance but you'll likely destroy pistons, rings, and cylinders. Your engine's manufacturer specifies a minimum octane rating. Go lower and your engine may knock or ping under load. It can put up with this for a while, but do it for too many tankfuls and carbon can build up in the piston ring's grooves and behind the rings. This forces ("jacks") the rings outward, scoring the cylinder or, worse, locking up the engine. While almost anything will run some of the smaller outboards (rated for as low as 84 octane), most engines require at least an honest 87 octane, with 89 preferred. For high-performance models, use 91 octane.
In Bearings We Thrust. Replace a prop on an outboard or stern drive and forget to put the thrust bearing back on first. Or if you have a few lying around, put on the wrong one. Without this washer-like bearing, the prop's hub rides hard against the gearcase each time you shift into forward. Check the price of a gearcase housing and you'll never forget your thrust bearing again.
Ultimate Screw-up. Don't read the instructions. I used to write these neglected pieces of literature and accepted that most people ignore them. Too bad, because our lives would be a lot easier, and our engines would last a lot longer, if we only took the time to refer to them every once in a while. As one manufacture put it, "The only thing we like to see wear out is the owner's manual."