How to Tune a Marine Engine
Thanks to the efficiencies of electronic ignition, tuning a marine engine has become simpler, less time-consuming and less expensive than ever before. In other words, do-it-yourselfers can more easily become involved with their engine's health care.
These days a marine engine tune-up consists of nothing more complicated than changing the spark plugs, air filter and fuel filters. When you buy the new spark plugs, review the manufacturer's chart that shows how to interpret different deposits. Also, if you notice that any of the spark plug cables are hard and cracked, they too must be removed and replaced. Because electronic ignition doesn't have points, a diy tune-up means never having to hook up a strobe light and adjust ignition advance.
If yours is an outboard or sterndrive engine, then a tune-up also includes renewing gear oil. Refill from the bottom plug, pumping in new lubricant until it oozes out the top hole. Replacing the top plug first will hold the oil in its reservoir long enough to slip in the bottom plug. Grease all the fittings on the outdrive and on the engine steering and shift mechanisms.
On four-stroke outboards, inboards and sterndrives, change the crankcase oil and filter. Draining the oil hot pulls more of the impurities out of the sump. Tip: Smear a dab of oil around the new oil filter's rubber-base gasket to help it seal more tightly. When you can do so without spilling, pre-fill the new oil filter before installing. That way the oil pump gets oil to the crankshaft journals a few revolutions sooner. The bearings will appreciate your attention to detail and reward you with longer engine life.
Gapless sparkplugs are plug-and-play easy. Right out of the box they're ready for installation. Just be careful not to over-tighten any sparkplug, because doing so risks stripping the threads. Classic side-gap sparkplugs are often preset. But be sure to double check the electrode to side gap clearance with a gapping tool. Owner's manuals specify a range, say from 35 to 40 thousandths of an inch (.034 to .040). Savvy technicians adjust gap closer to the smaller number because as an engine logs hours, its sparkplug gap widens (roughly .001 every 20 hours). A tighter initial gap means the engine stays in tune longer. For example, assuming 100 hours per season, an engine tuned with .040 plugs would be ready for a tune-up by the end of the summer, while one tuned with .035 gaps would be good for another year.
Fuel-injected engines benefit from an additional canned cure. Wax buildup on the injector nozzles causes fuel to dribble instead of mist. The fix: Dose the fuel with injector cleaner, or a tankful of high-octane gas. You're not buying octane, you're buying a superior additive package that unplugs fuel injectors and scrubs carbon from the combustion chamber. With either method, the before-and-after difference is dramatic.
Carbon buildup in the combustion chamber and transfer passages stifles efficient operation. Idle is rough, the engine stumbles on acceleration. The quick and dirty fix is to spray internal engine cleaner into the air intake (or carburetor on an older outboard, as shown here) until the engine (running at fast idle) dies. Let the potion work its chemical magic for about an hour. Remove the spark-plugs wearing leather or cloth gloves to protect against burning your fingers. Crank the engine for a few seconds to expel the liquid. Replace the plugs and restart the engine. Be prepared for a great billowing cloud of smoke as junk is expelled.
Tip: Stabilize the Fuel
Unless you're about to burn through a full tank, stabilize gas at the beginning, middle and end of the season. Modern blends' shelf life is only a few weeks. Incredibly, fresh pumped gasoline is already decomposing. It's not a problem in fuel-injected engines, but sour gas gums up carburetors in as short a time as a month. Finally, no matter which canned cure you use, read the label and strictly follow directions. Protect fingers and eyes with gloves and goggles.