So get out your toolbox now to prevent time-consuming and costly repairs.
When the weather is warm and sunny and the water is calm and cool, laboring over a hot, stubborn marine engine is the last thing on your agenda. Summer is for fun on the water, not work in the bilge. Unfortunately, no matter how thorough you were during commissioning, marine engines continue to require preventive maintenance throughout the season. Instead of procrastinating until a problem occurs before opening your toolbox, take time out for some engine chores with an idea towards preventing more time-consuming and costly repairs.
If you've logged more than 100 hours on your 4-stroke outboard motor, stern drive or inboard engine, consider changing the crankcase oil and filter. On the other hand, it's best to change the oil at the very end of the season so combustion by-products like sulfuric acid can't etch the crankshaft journals during lay-up. If you don't anticipate putting many more hours on your engine before decommissioning, you can delay changing the oil until then.
If you've noticed that your engine has begun to smoke a bit, which indicates that it's burning oil, an additive may help. Varnish deposits on oil-control rings cause them to stick, making them less efficient at scraping excess oil off the cylinder walls and dumping it back into the crankcase. Excess oil travels into the combustion chamber, where it burns and then exhausts as blue smoke.
Another source of oil-burning is worn valve guides, which result in oil being sucked up into the combustion chamber with every intake stroke. Worn valve guides require machine shop work, but stuck rings can sometimes be freed by dosing the crankcase oil with a product called TuFoil (available in discount and auto parts stores). TuFoil is a blend of motor oil and Teflon micro-powder, and I've seen it free up a set of rings in about 10 hours' running time. This quick fix will not work, however, if the crankshaft's main and rod bearings are worn and throwing so much oil onto the cylinder walls that even rings in good condition can't staunch the flow. And neither will TuFoil do any good is the pistons oil control rings are stuck in the lands or the cylinder wall is scored. The best way to prevent the problem in the first place is to regularly change engine oil so varnish doesn't accumulate.
If fuel economy has dropped since commissioning, and/or, if the engine is running rough, a simple tune/up may improve performance. The procedure need not be time-consuming or expensive. You can often get welcome results by just changing the plugs and cleaning the spark arrester, which should take less than an hour.
Begin by removing the spark plugs. Whether you clean them or install fresh ones - which many people favor because new plugs are so inexpensive - use the tight end of the recommended specification. In other words, if your manual suggests from .035 to .040 of an inch, adjust the clearance as close as possible to the smaller number. The reason for this is that marine engines' spark gaps widen about one-thousandth (.001) of an inch every 25 hours of running time. If you were to gap the plugs at the wide end of the specified range (.040 in our example), the ignition would begin to drift out of tune after only a few hours. By gapping "tight," your engine will stay in tune much longer. Weatherproof the spark plug cables by thoroughly spraying them with silicone lubricant. Not only does this keep the insulation supple, but it also reduces voltage leaks through moist air. Aerosol sprays are also good for removing moisture from electronic gear and wiring connectors.
Removing the spark arrester usually only requires loosening a single nut. Spray the screen with WD-40 or a similar solvent/lubricant. Direct the stream from the inside outward, so impurities are dispelled instead of jammed in. Never use gasoline or any other flammable liquids as a cleaner because of the fire and explosion hazard. Replace the spark arrester and your tune-up is complete.
If your engine still runs roughly, the problem may be water in the gasoline. Even smooth-running engines should be treated with fuel drier, whose active ingredient is alcohol. Alcohol is notoriously hygroscopic - it's like a magnet to water, absorbing and holding it in suspension. The alcohol itself is further held in suspension with gasoline, which flows through the carburetor or fuel injectors and is burned in the combustion chamber. Because today's fuel supplies suffer from abysmal stability, and degrade into gum and varnish so quickly, always stabilize gasoline before it's aged 90 days.
Never ignore a squealing drive belt. While the noise could mean nothing more serious than looseness, it could also signal that an alternator or water pump bearing is worn out and about to seize. Even if the belt has merely stretched and loosened, slippage will prevent the alternator and water pump from working to full capacity, and a dead battery or overheated engine could result.
One of the easiest ways to prevent outboard motor problems is to use the right fuel, which means avoiding 87-octane fuel if possible. Regular blends while the least expensive are not nece3sswarily the best choice for marine engines. They burn dirty and hot. It's not the octane per se that's the problem, but the inadequate base formula that causes carbon buildup. Mid-range, or 89-octane fuels, cost only a few pennies more per gallon and are well worth it.
Just as with stern drives and inboards, never let any gasoline age more than 90 days without treating it with fuel stabilizer to keep it from turning into gum and varnish. With outboards, it's also critical to treat gasoline supplies with fuel additives specifically designed to prevent and remove carbon and varnish deposits from the combustion chamber and piston tops. Outboard motors also benefit from burning nothing less than top-quality, marine-grade two-stroke oil. TC-W3 oil contains the necessary carbon-reducing additives.
If carbon has already gained a foothold in your motor, another canned cure can help. Back when by-the-book tune-ups called for removing the cylinder head(s) and side covers to scrape off thick carbon deposits, marine mechanics were loath to perform this labor-intensive task. Customers went ballistic when handed the big repair bill. Some mechanics took a lesson from Second World War B-29 bomber technicians, and squirted water into the carburetor of the big radial engines.
A better solution turned out to be spraying a can of internal engine cleaner down the intake, which is something that your motor may well profit from. The flood of carbon-killing juice stalls the engine, after which you should allow the chemicals to work for about an hour. Restarting results in a great billowing cloud of smoke. The engine chokes and wheezes for a few minutes, then smoothes out. For outboards in particular, this is an inexpensive and quick fix for rough idling.
To check both outboard and stern drive lower units, tilt them up and eyeball the propeller. Look for any, obvious damage to the propeller, like bent blades or pieces of metal missing from the tips. Just barely loosen the gearcase's bottom drain screw until the first drop of liquid drips out. If it looks like uncontaminated oil, you're in good shape. Simply retighten the screw. If the oil looks cloudy, or if water is any way evident, see a mechanic.
All freshwater-cooled marine engines should have their internal passages flushed as often as possible. Even lakes, rivers and streams contain corrosive elements. Some new stern drives and outboards come with a built-in garden-hose adapter. Older models use what looks like an oversized pair of earmuffs. The attachment fits over the water intakes and is held in place by spring steel arms. Hook up the garden hose for a few minutes, and the job is done.