America's Definitive Marine Engine Magazine

marine mechanic logo

Learn How to Tune an Outboard Motor
Stern Drive or Inboard Marine Engine

 Mercury Marine certified mechanic working on an outboard

Photo courtesy Mercury Marine Docklines

By Binnie Smith

Keeping your outboard or inboard in tune is easier than ever because of technological advances.

Boating is significantly more pleasurable when your engine is in perfect tune - when it fires with the first turn of the key, when idling is smooth and when acceleration is razor-sharp. Keeping your outboard or inboard in tune has become easier of late because of technological advances. For example, most ignition systems nowadays are breaker-less, so gap and timing settings are things of the past. Basically, most of the work required to maintain an engine can be accomplished by even a moderately skilled Do-It-Yourselfer. With that in mind, here are some tips that will improve your wrench bending skills.

Before Tuning an Engine, First Check Compression. An Engine With Low or Uneven Compression Will Never Run Right

Begin at the beginning. If the starting-battery voltage is low, then cranking may be slow, preventing a clean start. A shortfall of as little as one volt can make a big difference. If your boat remains idle for weeks on end, you should have your marina periodically trickle-charge the battery. If the battery is more than three or four years old, and has suffered neglect over that time period, you should consider replacing it. It's also important to maintain the battery. Keep the terminal posts clean of corrosion that can bleed voltage. A simple coat of grease holds the green stuff at bay. Periodically wash the battery top, because dirt can also rob voltage.

Next, make sure the airflow into the carburetor or EFI throttle body is unrestricted. On stern drives and inboards, this means removing the spark arrestor. Spray its metal filtering element from the inside out with a copious stream of carburetor cleaner, available at auto parts stores for a couple of dollars. Spraying from the inside out forces debris away from the engine, instead of towards it. The solvent removes stubborn bits of oil, varnish and other contaminants.

Now attack gum and varnish that may have accumulated inside the carburetor throat and on the throttle and choke plates (or on an EFI motor's throttle body). Also, shoot solvent on the linkage. Remember to re-lubricate the linkage when finished.

Freer breathing

It's important to make sure the choke is operating correctly. When the engine is stone cold, the choke plate must be closed. Conversely, when the engine has warmed up, the choke plate should swing wide open. If it doesn't, probable causes include stuck linkage, an incorrect adjustment, or, on an electric choke, a broken wire or blown fuse.

Outboard motors differ from inboards and stern drives in that, instead of a having a flame arrestor, they filter intake air through a foam element. Best bet here is to consult your owner's manual for the correct procedure.

Decarbonize outboards

Rough-running outboard motors are Often suffering from carbon buildup. The two most common causes of build-up are cheap gasoline and inferior two-stroke oil. Either cause can be easily remedied. The quick fix is to run the engine until it's warm, then shoot internal engine cleaner into the intake. Faithfully follow the directions on the aerosol can for best results. Essentially, this treatment acts like a lye-based oven cleaner, dissolving carbon from piston tops, combustion chambers and exhaust ports. In the '60s, with lower quality oil and heavy oil-to-gas ratios, it was common practice to remove cylinder heads and exhaust port covers to scrape away carbon obstructions.

Prevent carbon accumulation on your outboard by lubing with TCW-III, or adding a carbon-fighting additive to the fuel. Another important step is never to burn 87 octane gasoline. This blend is typically dirty beyond belief. In fact, it's so dirty that poor performance can often be cured simply by stepping up to 89 octane. It's important to know this is not an octane problem, but a fuel quality problem caused by modern gasoline-refining methods.

Internal engine cleaner and higher-octane fuel can also produce good results on afflicted inboards and stern drives, although most four-stroke engines do not react as negatively to junk gasoline. However, if your gasoline stern drive or inboard "diesels" (keeps running for a few moments after you shut it down), try topping off with a higher-octane fuel. If the dieseling doesn't clear up, perhaps the spark plug's heat range is too high, or a carbon ember glowing red hot on the piston top is igniting the fuel. Try the internal engine cleaner.

Change the spark plugs

Because they're so inexpensive, it's foolish to reinstall an old one. I routinely replace plugs once a year. Be careful not to drop one, because an insulator crack - even though it may not be visible - will keep a plug from performing up to its full potential. It may even kill it.

sparkplug wet and dry Double-check the spark plug heat range. In other words, take the time to consult the owner's manual. I often find the wrong plugs in a motor, and the damage has already been done: overheating, burned pistons, or simply poor performance. Whether they're too hot or too cold, the wrong plugs can wreak havoc.

When replacing the plugs, gap towards the tight end of the spectrum. In other words, if the gap should be from .030 to .040, gap as close to .030 as possible. As engine hours accumulate, the gap opens, so this method keeps the engine in tune longer. Never make the mistake of over-tightening spark plugs. Stripped threads are a real danger. Never coat the threads with any substance, because it will interfere with the proper transfer of heat out of the plug and into the coolant.

Spark plug wires deserve more attention than they get. The extreme heat that surrounds marine engines makes the wires more prone to cracking, which is a problem in damp air because moisture provides a path to ground, bleeding voltage. With fewer volts jumping the gap, the spark is weaker and the combustion of gasoline is, in turn, less robust. Symptoms include rough idle, stuttering and less top-end speed. Spotting faulty wires is easy. All you need is darkness and either damp sea air or a spray bottle of water. Run the engine, being careful to keep clothing and body parts away from belts, pulleys and hot manifolds. If the air is damp, and the cables are cracked, you'll see blue arcs pin-pointing the flawed insulation. If you're not sure if the air is sufficiently moist to provide the necessary ground, spray a mist around the suspected wire.

Keep your cool

Be advised that overheating causes most marine engine problems. For that reason, it makes good sense to monitor engine-operating temperatures. If you don't already maintain an engine log, begin one this season. If the engine is equipped with a coolant temperature gauge, routinely record the measurements. These benchmark readings will come in handy down the road. If you suspect a problem, you won't have to guess - you'll know whether the temperature has been slowly creeping upwards, or if it has suddenly spiked.

Similarly, it's not a bad idea to at least occasionally Pre-flight the cooling system. Closely examine belts and hoses. Look for flaws such as frayed drive belts and hardened hoses, telltale signs that failure is imminent. On closed-cooling systems, check the coolant level.

squeezing a coolant hose looking for weakness deflecting a drive belt looking for slack

If your boat does not have a temperature gauge, and you don't want to go to the expense and trouble of installing one, there are a number of simple solutions. The quickest way to tell that a power head is overheating is to touch it (please be careful). If you can almost, but not quite, lay your hand on the cylinder block or head, then the temperature is within limits.

coolant level sea strainer

However, if touching the metal casting would scald your hand, or if droplets of water flicked on the surface sizzle and evaporate, then there's something amiss. Water boils at about 210 degrees F - a higher temperature than that at which a healthy engine operates. A more sophisticated approach involves the use of temperature crayons. Available for a couple of dollars at auto parts stores, these sticks melt at different temperatures depending on their ratings, and will tell you at what temperature a power head is running

No matter how strongly an engine runs, it does no good if propeller pitch isn't dialed in to match your style of operation. Banzai acceleration demands different pitch than does maximum top speed or fuel economy. There's more: The exact wheel the factory specified for your boat may no longer hang on the propeller shaft. Sometimes careless technicians mismatch customers' propellers during fall lay-up or the frenzy of spring commissioning. If you're not up to speed on propellers, talk to whomever works on your boat and ask him or her to determine whether you have a speed or power prop. Talk over what kind of performance you expect from your boat and the wisdom of pitching up or down.

On stern drives and inboards, check the valve cover gaskets for leaks. Lay the correct-sized socket on each bolt and test its tension. Don't make the mistake of over-tightening these bolts, or you could warp the seal and inadvertently cause a dribble of oil where one would not have otherwise existed. Naturally, if you should find any loose bolts, tighten them. If that doesn't fix the leak, replace the gasket.

Compression Check

If your engine purrs like a tiger, odds are good that the rings are sealing adequately. However, rough idling and sluggish acceleration are indications that an engine may be suffering the ravages of wear. Moreover, weak compression, or compression that varies by 10 or more points between cylinders, means that the engine will never run right. You can change the plugs and adjust the carburetor daily, but it won't make a difference.

For this reason, it's good practice to check compression. There's no need to buy a compression gauge for the few times you're likely to use one, however. Rental is inexpensive. The procedure itself is straightforward.

Warm the engine, then shut it down and remove all the spark plugs. Crank the engine, recording each cylinder's reading. Compare the results with the specifications listed in the repair manual. Another source for this information is your service technician. As for the data you'll get, if readings are low on adjoining cylinders, it usually indicates a leaking head gasket. Squirt about an ounce of oil into any cylinder that reads low, then crank the engine through several revolutions. Test again. If the reading climbs higher, the rings are worn. Otherwise, the valves aren't sealing.

Finally, don't forget to check the fuel system. Replace fuel filters at least once a season. If you want to be really on top of things, consider installing a filter/water separator.