Spark Plugs

Marine Engine Spark Plugs Resources and Information

Changing your spark plugs is an essential part of outboard engine maintenance. That’s why PartsVu stocks a wide variety of spark plugs from the most trusted marine products manufacturers in the industry. Browse our complete selection to find the spark plug that matches your outboard engine’s exact specifications. 

We also regularly publish helpful resources over on our blog to make servicing your outboard a breeze. The following are a few: 

• Marine Spark Plugs – Everything You Need to Know
• Mercury Spark Plug & Compression – 4-Stroke 100-Hour Service
• Yamaha Spark Plug & Compression – 100-Hour Service

Looking for more products for your outboard or boat? Shop PartsVu’s parts and accessories and aftermarket parts, or shop our outboard spark plugs by category, including: 

• Spark Plug Wire Sets
• Spark Plug Accessories

Outboard Engine Spark Plugs Resources and Information

How do marine spark plugs work?

Marine spark plugs supply the spark that ignites the air-fuel mixture, creating the explosion that allows your boat’s engine to produce power. Spark plugs generate an arc of electricity across two leads which are not touching but close enough together that electricity can jump the gap between them.

How often should I change my marine spark plugs?

Boaters debate the answer to this question. Some boaters are adamant about changing their spark plugs every year or 100 hours. PartsVu recommends religiously inspecting your spark plugs annually or every 100 hours, replacing them if they are fouled or damaged, and always replacing them every 300 hours.

What should I look for when inspecting my spark plugs?

Inspecting your spark plugs relays essential information about your engine’s health and can help you address small problems before becoming big problems. Specifically, the color of the insulator firing nose tells you a great deal about your engine’s overall operating condition. Generally, a light tan or gray firing nose indicates that the spark plug is operating at optimum temperature and that the engine is in good condition. 

Suppose the insulator firing nose contains either wet or dry dark color deposits. This indicates an overly rich operating condition, that the engine is operating too cold, a possible vacuum leak, low compression, overly delayed timing, or a spark plug gap that is too large.

If the deposits are wet, this points to a failed head gasket, oil control from piston rings, or an extremely rich operating state depending on the specific liquid at the firing end.

If your engine is overheating, spark plugs may exhibit a bright white glazed or glossy insulator firing nose or include metallic speckling. This coloration can result when your engine operates at a temperature range that is too high or runs in excessively lean conditions. Additionally, improperly tightened spark plugs may contribute to spark plug overheating.

The root causes of spark plug fouling or overheating must be corrected quickly to prevent engine performance deterioration and possibly severe engine damage.

How do I change my Mercury 4-stroke outboard spark plugs?

For most applications, the spark plugs come pre-gapped, but it is advisable to re-check gap to make sure that plugs were not damaged during manufacturing or shipment.

While removing the old spark plugs, keep track of which plugs come from which cylinders.

• Note if the plug appears to be wet from water, oil, or gasoline.
•  Note if the plug exhibits evidence of heavy carbon deposits or other fowling as well as any rust – all indicative of problems.

If you were to take your engine to a dealer for your 100-hour service, they would perform a compression test by connecting a compression gauge to the individual cylinders and using the starter motor to turn over the engine. By measuring the engine’s ability to pressurize the cylinder, you can glean information about the condition of the engine internals. To perform this test at home, you will either need a remote starter button, or a friend who can turn the ignition key with the kill lanyard removed.

This cranking with the ignition disabled will also prime the fresh oil around the block, reducing friction at startup after the service. Repeat testing for all cylinders and compare results. Readings will vary from gauge to gauge, so the test is more about measuring the difference in readings with ideal results being less than 10 percent variance.

After the compression test is completed – or skipped – you can now begin installing your new spark plugs. For ease of future removal and to reduce the risk of thread damage to the cylinder head, apply a tiny amount of anti-seize compound to the threads of the plug. Use care to avoid contacting the electrode or porcelain center of the plug. Thread the plugs into the cylinder head by hand using caution not to cross thread.

Most modern spark plugs use a crush washer that will press down and provide a tight seal as you tighten. Full torque is between 18-21 ft. lbs. Reinstall the plug wires or coils in the same order they came off. Mercury’s service manual recommends using Loctite 271 (Red High Strength) on the bolts that hold the pencil coils over the spark plug, but we find Loctite 242 (Blue Removable Strength) might be a better choice.

How do I change my Mercury 2-stroke outboard spark plugs?

While removing the old spark plugs, keep track of which plugs come from which cylinders.

•  Note if the plug appears to be wet from water, oil, or gasoline.
•  Note if the plug exhibits evidence of heavy carbon deposits or other fowling as well as any rust – all indicative of problems.

If you were to take your engine to a dealer for your 100-hour service, they would likely perform a compression test by connecting a compression gauge to the individual cylinders and using the starter motor to turn over the engine. By measuring the engine’s ability to pressurize the cylinder, you can glean information about the condition of the engine internals. To perform this test at home, you will either need a remote starter button, or a friend who can turn the ignition key with the kill lanyard removed.

This cranking with the ignition disabled will also prime the fresh oil around the block, reducing friction at startup after the service. Repeat testing for all cylinders and compare results. Readings will vary from gauge to gauge, so the test is more about measuring the difference in readings with ideal results being less than 10 percent variance.

After the compression test is completed – or skipped – you can now proceed with your Mercury 2-stroke spark plugs and compression service by installing your new spark plugs. For ease of future removal and to reduce the risk of thread damage to the cylinder head, apply a tiny amount of anti-seize compound to the threads of the plug. Use care to avoid contacting the electrode or porcelain center of the plug. Thread the plugs into the cylinder head by hand using caution not to cross thread.

Most modern spark plugs use a crush washer that will press down and provide a tight seal as you tighten. Full torque is between 18-21 ft. lbs. Reinstall the plug wires or coils in the same order they came off.

How do I change my Yamaha outboard spark plugs?

Begin this step of your Yamaha 100-hour service by removing the plastic shrouding over your spark plugs or ignition coils. Yamaha engines come from the factory with NGK brand spark plugs, and PartsVu carries a wide assortment of these OEM plugs. In most applications, the spark plugs come pre-gapped, but it is advisable to re-check gap to make sure that no plugs were damaged during manufacturing or shipment. Again, the information in your owner’s manual is helpful – or you can consult our Yamaha Engine Specifications and Capacities Chart. 

As you remove the old spark plugs, keep track of which plugs come from which cylinders. Note if the plug appears to be wet from water, oil, or gasoline. Note if the plug exhibits evidence of heavy carbon deposits, other fouling, or rust – all indicative of other problems. If you were to take your engine to a dealer for 100-hour service, this is the point at which they would perform a compression test by connecting a compression gauge to the cylinders individually and using the starter motor to turn the engine over and check its ability to pressurize the cylinder. If you are going to perform this test at home, you will either need a remote starter button, or a friend who can turn the ignition key with the kill lanyard removed. This cranking with the ignition disabled will also prime the fresh oil around the block, reducing friction at startup after the service is complete. Repeat testing for all cylinders and compare results. Readings will vary from gauge to gauge, so the test is more about measuring the difference in readings with ideal results being less than a 10 percent variance.