How to change outboard motor spark plugs

Changing your outboard motor spark plugs often is essential to its performance

You should be changing your outboard motor spark plugs every year or every 100 hours whichever comes first. If you don’t your outboard motor performance and fuel efficiency can suffer. So to help keep your outboard motor running great we will show you step by step how to change spark plugs in outboard motors. 

Tools you will need to change outboard motor spark plugs

1. Socket wrench.
Preferably a torque socket wrench, but either a regular socket wrench or a torque socket wrench will work. More on this in step 6 below.

2. Socket that fits your spark plugs properly. 
There are many spark plug specific sockets that work great that you can use. Or you can also use a deep socket that fits your spark plug nut pattern. More on that below.

3. A socket extension may also be needed.
Sometimes getting your socket to reach the spark plug can be tricky with all the other parts on the motor getting in the way. Often you may need to use a 3 inch or even a 6 inch socket extension bar to reach the spark plugs properly and have enough clearance to use the socket wrench without hitting anything on the motor.

4. Sometimes a swivel socket extension may also be needed.
Usually this is not needed but some outboard motors have a spark plug in a position that makes it difficult to reach with a straight extension. Generally the lowest spark plug on the outboard motor is the one that gives people trouble while trying to reach it. As the available space is not enough to access with a straight extension. A swivel extension allows the socket to pivot at an angle allowing better access.

Step by step Directions on how to replace spark plugs on your outboard engine

Step 1. Turn off the battery switches and remove the cover on the outboard engine

It is very important to disconnect the battery or batteries from the engine or engines that you are working on to prevent electric shocks. Most boats you can simply turn off the battery switches to achieve this.

You should also work on an engine that is cool to the touch to prevent being burned as some parts of the engine get very hot. Spark plugs in particular get very hot after the engine has been running. Especially if it has been running at high RPMs. So use caution.

Step 2. Locate your spark plug wires and remove them from the spark plugs

Once you find your spark plug wires and spark plug boots carefully pull them off the spark plugs. If however it may be unclear which spark plug wire might go to which spark plug after removing the wires, then before removing the wires mark them with blue tape and write the cylinder number on the tape. You can rotate the wires where it attaches to the spark plug 90 degrees to get a better grip on the spark plug boot if needed. Make sure you pull only from the thickest part which is the spark plug boot that goes onto the end of the spark plug.

Pulling on the wire its self can lead to damaging the spark plug wires. If you damage the wires your quick replacement of just the spark plugs will now become a much bigger project. So take your time and be careful.

Step 3. Use a socket wrench to remove the spark plugs.

Use the appropriate sized socket for your spark plugs and a socket wrench to remove the spark plugs.  Below is a gallery on how to pick the right sized socket. With your new spark plugs on hand take one out of the box and test it inside various sockets until you find one that fits perfect.

In this example a 18 mm deep socket fits the NGK DPR6EB-9 spark plugs perfectly. This spark plug model is used on a lot of different outboard motors. And is one of the favorite spark plugs used on many Yamaha outboard engine models. 

Step 4. With Spark Plugs removed Inspect the spark plug holes and old spark plugs

This is a quick but important step that is often overlooked.  Take an extra minute to look at the spark plugs. Check to see if any of the diodes are broken, or if any of the spark plugs look different than each other. If there is a major difference in the color between the spark plugs. Or if there is a major difference in the amount of build up on the spark plugs this could mean other issues are happening in the engine. This is one good way to check for possible future issues so you can resolve them now before they become worse.

For example if you look at the spark plug and notice the center electrode porcelain is very white it could indicate that there is a intake air leak or possibly an issue with the carburetion for that cylinder. 

Inspecting the spark plug holes where the spark plugs thread into the engine is also important because you can check for corrosion. Especially in salt water environments the beginning of the threading where the spark plug goes in can have a tendency to start to corrode. If it does make sure to remove any corrosion before continuing making sure not to get anything into the spark plug threaded hole.

Step 5. Set the spark plug gap OR at least check the gap compared to the other New spark plugs

99% of the time you will not have an issue with the spark plug gap being off. So generally you will not need a spark plug gap measuring tool. As manufacturers now set the spark plug gap on each spark plug model number the same. In the past one spark plug model number might be used in various engines needing different spark plug gaps. Now there are so many model numbers the gap is set specifically to the model number. 

So if you don’t have a spark plug gap tool don’t worry. Just carefully check the gap between all of the new spark plugs. If one was damaged while shipping which caused the spark plug gap to change you will be able to notice it when comparing it to the others. If one is off either discard and use another, or get a gap measuring tool to make sure the gap is set right by either widening the gap or narrowing it down.

Out of habit I always check the spark plug gaps during routine maintenance. But I can’t remember the last time I found one where the gap was incorrect. Even so I still continue to do it, as I would rather know if it is correct before installing them.

Refer to the gallery below on checking the spark plug gap before installing new spark plugs into an outboard engine.

 Step 6. Screw all spark plugs back in only finger tight first, then use a torque wrench to finish

First screw in all spark plugs only finger tight into the spark plug holes. Then come back with your torque socket wrench and torque the spark plugs in at the amount listed in your outboard motor manual. In this example the spark plugs are supposed to be torqued to 12.5 ft-lb or 17 Nm for this 40 HP Yamaha engine. With the torque wrench use what ever socket fits your spark plugs. In this case a 18 mm deep socket was used as it fits the NGK DPR6EB-9 spark plugs perfectly used on this 40 hp Yamaha FET.

If you do not have a torque wrench

If you only have a regular socket wrench a good estimate to reach the correct torque of 12.5 ft-lb or 17 Nm is to screw the spark plugs in until they are finger tight. Then use a socket wrench to go 1/2 turn past finger tight when installing a new spark plug. For re-installing a previously used spark plug go only 1/12 turn past finger tight.

1/2 turn past finger tight is the estimated rotation amount needed for reaching a torque of 12.5 ft lb or 17 Nm while using a brand new NGK DPR6EB-9 spark plug and other similar spark plugs. If your engine requires a different torque setting on your outboard engine spark plugs please follow the guidelines in your outboard engine manual. If you do not follow the manuals guide lines you can strip the threads on the engine block.

For when using a torque wrench below is a gallery on how the torque wrench is set for this particular engine to reach 12.5 ft lb / 17 Nm torque on the outboard motor spark plugs. And what type of torque wrench was used. This one is from Tekton and is the 3/8″ version which allows torque settings down to 10 ft lb. 

While using the torque wrench go slowly so that when you feel the “click” indicating you reached your desired torque limit you can immediately stop turning the handle. As the click only indicates that you reached the set torque limit. It does not stop you from continuing, it is only an indicator.

Step 7. Install the spark plug wires onto the correct spark plugs

Take the corresponding wire to each spark plug and push the spark plug boot onto the spark plug end. You will feel a type of “click” when the spark plug boot is seated correctly onto the spark plug end. If you do not feel a “click” the spark plug wire is probably not seated onto the spark plug correctly yet. When in doubt carefully remove the spark plug wire and boot and start over until you feel it seat properly with a type of “click”.

Generally on outboard engines figuring out which spark plug wire goes to which spark plug is fairly simple to follow as the spark plug wires are usually tied in a way onto the engine that places the wires close to their corresponding spark plugs. As mentioned earlier is step 2 if it is unclear which spark plug wire might go to which spark plug before removing the wires in step 2 mark the wires with blue tape and write the cylinder number on the tape.

Take your time to make sure you do place the correct spark plug wire on the correct spark plug. Otherwise you can cause the engine to run rough or run not at all as the timing will be off which can also lead to damage.

Now you are ready for some fun on the water!

After replacing your spark plugs on your outboard motor take your boat for a test ride if possible. If you can’t test your boat until you trailer it to the boat ramp. Just make sure you test the engine at neutral rev at the boat ramp dock before venturing out to make sure everything is working right before casting the lines off.

From all of us here at HelpfulCaptain.com we hope you enjoy your time on the water!