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In this section, we will be referring to 8 cylinder engines, but the same basics apply to any marine inboard engine unless otherwise specified. The ignition system of a marine engine is fairly simple for earlier models, and fairly sophisticated on later models. Regardless of which you have, don't let ignition problems fluster you. We will take a look at both early and late model ignition systems, and take some of the mystery out of troubleshooting, along with how to correct ignition problems.

Most early model marine engines are all typically the same when it comes to ignition systems. They consist of the ignition switch, a coil to store and release enough energy to fire the spark plugs, distributor to distribute the spark to the correct cylinder at the correct time, and the spark plugs which fire to cause combustion in each cylinder. Since design is very simple, there is not all that much that can go wrong, and not be easily fixed.

Ignition switch

To start things off, lets look briefly at the ignition switch, figure 1. Everyone would know what this device basically does, but can sometimes be overlooked when troubleshooting an ignition problem. Simply make sure that the key solidly clicks to each position without sloppyness involved. If you cannot distinctly feel the "click" as you rotate the key to each position, replace the switch. Make sure wires on terminals are tight and free of corrosion. If nothing unusual can be found with ignition switch, then it most likely is in good working order.

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Ignition coil

Next, we look at what happens when you turn the key switch to the on position. This energizes the ignition system and makes things start to happen. First thing that gets the "juice", is the resistor mounted near the ignition coil if equipped with external resistor, or the ignition coil if equipped with internal resistor figure 2. If your motor seems to not be getting any fire, this would be the place to check first. With a voltmeter, and key switch in the "on" position, check to see that + side of coil(or purple wire) has a full 11 to 12 volts to it. If so, then switch and wire are in good shape. If not, trace purple wire until you find the break. Unfortunately, there is no sure fire way to test the coil without replacing it, so if the rest of the ignition is in good shape, that might be the last thing to try in the case of "no fire". The ignition coils job, is to acquire and store electrical current, amplify stored current upward to around 15,000 volts and then release the energy to fire each spark plug.

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Distributor

From the coil, we go to the distributor. The distributor is a very uniquely designed part that serves many functions. Its main job is to tell each spark plug when to fire, and it also self adjusts spark advance to keep the motor at peak performance. Early models use a conventional contact point system figure 3. Later models have changed to "HEI" or electronic ignition figure 4. Both are serviceable, but look and operate quite differently when viewing the internals. The distributor needs to be checked periodically for being set at the correct timing. Timing is essential when supplying fire to each spark plug at the correct moment. Refer to a manual, or email us for the specified timing setting for your particular engine application.

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First lets give the conventional point type distributor a look, figure 3. Upon removing the distributor cap, you will see the rotor, points, condenser, and a black wire coming from the "-" side of the ignition coil to the points. This wire must be in good shape with no frays or actual contact to any ground source. If it has a dead short to ground, ignition will not fire. Also, if any corrosion what so ever is seen inside of distributor, replace the parts as required. Do not attempt a simple cleaning of the parts, due to once corrosion starts on a part, it will reoccur no matter how much cleaning is performed. As the distributor shaft rotates, the rotor also turns and the tip makes slight contact with the lugs in the cap. This is where distribution takes place. The points are attached to the point plate, with one end tip riding on the cam lobe. As the cam lobe rotates, the points open and close accordingly. At the precise instant that the points are opened, a break in the current occurs, and causes the ignition coil to output the stored energy. The condensers main function is to balance current flow and actual reverse the process through the points. The point gap on most engines is about 19, or .019 thousandths of an inch.

Points replacement (tune-up)

Needs for replacement of conventional type distributor parts include burnt points, engine will not start or rev to full RPM(points worn or set at wrong gap), or any signs of corrosion, including distributor cap. Make sure key switch is in the "off" position when performing the following steps, or you may end up with a new hairdoo. Close inspection of the points should be made by gently opening point gap with a screwdriver. Observe point surfaces to see if they look excessively burned, corroded, or pitted. If any of these are present, replace the points. The points should be gapped with the cam lobe end tip of points on the very top of the cam lobe point so that a feeler gauge can slide in between freely, but with a VERY slight resistance. After gapping points and re tightening set screw, rotate motor a few revolutions, then recheck gap to ensure nothing moved. It is recommended that when replacing points, you simply install a complete tune-up kit which includes points, rotor, and condenser. This makes sure that all parts are in good working order. It is also recommended that an extra tune-up kit along with necessary tools for replacement be kept in the boat at all times. If one of those parts fail, it is very simple to replace them, rather than having to be stranded in the middle of nowhere or towed back to shore.

HEI ignition

Now lets take a look at the "HEI" or electronic type distributor figure 4. With cap removed, this distributor looks rather plain inside due to condensing the parts into what is called a magnetic sensor. As the rotor turns in this application, a magnetic connection is opened and closed to work much as points do in the point type distributor. Also, located externally in this system is a non serviceable ignition amplifier, that controls the magnetic sensor. There is not much service to do on this type of distributor other than making sure everything is clean and corrosion free. The sensor however, can go bad, but it is a pretty rare case that this happens. The improvements in the "HEI" system are superior to that of the point type. Should you have a problem with no fire to the spark plugs with this system, the diagram of figure 5 will guide you step by step on how to determine the problems with an "HEI" ignition system.

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Spark Plugs & Wires

Next in either system are the spark plug wires and spark plugs. Plug wires should be checked at least once a season for cracks or frays in insulation or boots. Sometimes even a plug wire that looks to be in good condition can cause a problem. If you experience a dead cylinder condition(dead miss) or an intermittent miss, it is possible that there could be current leakage in one or more plug wires. The easiest way to find this without getting your socks knocked off is to start and run motor at night, or inside a garage with lights off. It is easy to see spark jumping to ground from a plug wire in a dark engine compartment. If any plug wires are found to have the preceding problem, the entire set of wires should be replaced, and it is recommended that the entire set be replaced every two years, regardless of usage.

"Oh my gosh!, I forgot to mark which plug wire went where!" Not to fear, as long as you know which lug was for the number one cylinder on the distributor cap, most engines have the firing order cast into the intake manifold. Simply follow the firing order in the correct direction of rotation on the distributor cap. "..but whats the correct direction of rotation?" That depends on the engine make and design, and whether it be left hand rotation or right hand rotation. The diagram below in figure 6, will offer the specified firing order for your specific motor.


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Spark plugs supply the spark internally to each cylinder causing combustion of air/fuel mixture as piston rises and compression is at its maximum. Spark plugs should be replaced about every two years as well, depending on usage. Reasons for replacement can be anywhere from fouling do to oil leaking past piston rings or valve seats, to damage caused by pre-ignition(firing prematurely, incorrect timing). The center porcelain tip of the threaded end of the spark plug should be a medium to dark brown color after about 10 hours running time. If it is white or gray(pre-ignition) after 10 hours, you need to check the timing. This could be signs of severe damage in process. If it is black containing carbon deposits, it means that excessive fuel is entering the combustion chamber. Black, or "fouled" plugs, can result from carburetor problems, or by insufficient compression due to worn piston rings. In either case, a decrease in performance should be noticable. The gap of spark plugs should be set to approx. .035 thousandths of an inch in most applications. Spark plugs can be replaced with any brand you prefer, but make absolute sure that they are the correct heat range and type for your application. Installing the wrong heat range or type can cause severe engine damage. In certain circumstances, you may increase heat range of spark plugs by one to two steps, but never more than that.

If you should have any questions about your ignition system that may not be answered in this section, feel free to email us. We will gladly help you in any way that we can.