You are getting ready for the lake and preparing the boat for an endless summer of lake bound journeys.  From sunrises to sunsets on the water everyone wants their boat to start up quickly and run smoothly every time they fire up the engine.  Inboard or outboard most boats have one thing in common.  They all have spark plugs.  What I want to help you answer is the question of if your boat’s spark plugs need to be changed before you hit the lake this spring.

My first suggestion is an owner’s manual for your boat and an assessment of your skills.  It is completely possible that this job is above your pay grade and far too challenging for you to undertake.  However, you are reading a blog post on how to change your boat’s spark plugs.  If you have an interest in learning a new skill we at Lake Fun salute you.

Assuming you have disconnected your battery and fuel line it is time to take the spark plug boot off the spark plug.  Go ahead and find a spark plug socket and loosen each plug by turning it counterclockwise.  Remember, righty loosey.  Lefty tighty.  Just kidding.

After you have the plugs out it is time analyze them for other issues that might be present in your engine.  I will explain below using the color or discoloration of your spark plugs electrode.

Light grey to tan color with a few deposits means that you have won the boat lottery this spring.  Find a replacement spark plug that is exactly the same as the plug you removed and replace it.  Now you are ready to go on the water.

Another scenario is if a dark deposit has bridged from the edge of the spark plug to the inner electrode.  This can be caused by a few things:

  1.  Wrong Engine Oil
  2. Carbon build up in the cylinder of an older engine
  3. Incorrectly mixing your fuel and oil ratios
  4. Keeping your motor trolling or idling for long periods of time and shifting to immediate high speed operation.

In this case I would change not only the spark plugs, but also the way you treat your boat.  Every item on the list except for problem #2 can easily be fixed.  Next spring when you remove your spark plugs the same issue should be gone.


The next case we will see is what will look like the top of the spark plug has been sprayed with tiny black dots.  Almost like someone took a can of spray paint and briefly sprayed the top of it.  This will typically still have a wet feel to it and may wipe off easily if you brush it with a paper towel.  Possible causes might include.

  1.  Idling for long periods
  2. Use of the wrong spark plug
  3. Carburetor too rich

Idling for long periods can be solved by changing the way you operate your vessel.  If your spark plug range is too cold you can ask about moving to a higher heat range.  I tend to suggest sticking with what the manufacturer originally recommended.  Is that what came out of your boat?  If not, consider switching back to it.  If your Carburetor is running too rich consider an adjustment.  A carb adjustment is beyond the scope of this tutorial, but a marine engine specialist should be able to make the adjustment for you easily, just schedule an appointment.

The next example is a spark plug that looks like it has burnt, bubbling, and damaged insulator.  This can point to serious problems.  The first items you should check.

  1.  Is your spark plug range too hot?
  2.  Is your water pump or impeller working properly?
  3.  Is your cooling system blocked
  4.  Is your propeller sized and installed correctly.

In this case you should not replace your spark plugs and do a thorough search for what is causing the excessive heat inside of your engine.  Failure to correct this problem could result in major damage to your boat’s motor.  Start with the easy items and work your way down.  Did you modify any of the items in the list above in recent years?  If so, consider going back to the factory specs.


Hopefully you removed your spark plugs and found a nice tan color electrode like I did.  I replaced my 3 plugs with 3 fresh ones from the local auto parts store and took my boat down to the boat ramp to test it out.  It started up perfectly and I can check one more item off my list for my lake preparations.  Many people will ask, “Should we reinstall the old spark plugs if they look ok?”  It is completely possible to use some spark plugs for another season, but my suggestion would be not to chance it.  A better idea would be to place them in your boat’s glove box to use in an emergency as a spare.  Then go spend the $10-$20 to replace your plugs and have the piece of mind of a fresh set of plugs in your boat this summer.