How to Test Outboard Motor Spark Plugs 

An outboard needs three basic things to run: fuel, air, and a spark. The air is all around us and the fuel is in the tank, but a lot of stuff has to happen in order to get that spark. 

The combination of air, fuel and ignition culminates at the spark plug. If your outboard won’t start, and you’ve run through some obvious possible causes (pulled kill switch, no gas in the tank, loose connections, etc.), you need to dive a little deeper. Here’s how to inspect outboard spark plugs to make sure they’re doing their job.

Tools Needed - Outboard Spark Plug Testing

CDI Electronics spark plug tester

Buy outboard spark plugs

Outboard Ignition System Electrical Current Test 

Step 1. Test for electrical current to the spark plug by pulling off a spark plug wire and connecting it to one side of the spark plug tester. Next, place the spark plug tester at the top of the plug or a piece of unpainted metal that can act as a ground. 

Outboard spark plug test electrical current

Step 2. Turn over the engine to see if you can see a spark jump across the gap in the tester. If it doesn’t produce a visible current, it's not getting any power or if it’s dim, it may not be getting enough power. 

Outboard spark plug testing

A bright spark is an indication that you have good current running through the plug wire. Move the gap out further to see how strong the spark is, and repeat the process if you have a multi-cylinder engine.

Evaluating the Spark Plug on an Outboard

Step 1. Remove the spark plug with a deep-well socket to inspect it. 

Outboard spark plug evaluation

In a 4-stroke, oil on the end of the spark plug can mean oil is getting past the rings, gaskets or even a valve seal and it’s getting down into the combustion chamber. In a 2-stroke, it can mean there’s too much oil in the fuel mixture.

Outboard spark plug inspection

You may see carbon on the electrode, which can mean the motor is set to run too rich. It can also mean there’s something going on inside of the intake that’s restricting the air from getting down into the cylinder. This changes the air-fuel mix and leaves a rich condition, and may suggest that something is obstructing the air intake.

Outboard spark plug inspection running rich

On the other end of the spectrum, you might see white on the tips of the plugs. That can mean the motor is running way too lean. A lean-running condition can take a major toll on the motor, as it tends to run a lot hotter this way.

Outboard spark plug inspection lean

Step 2. Check the spark plug gap with a feeler gauge to make sure it’s set just right. They’re usually set where they need to be, but with the plug out it’s a good time to double-check it anyway.

Outboard spark plug inspection feeler gauge

Testing for Spark

NOTE: Before checking for spark, make sure you’re in a place with no flammable fumes. Perform the test as far as possible from the open spark plug hole. Even the unignited fuel in the cylinder can produce a fire hazard.

Step 5. Reconnect the plug wire to the spark plug with the plug still removed from the engine. 

Outboard ignition system spark plug test

Step 6. Hold the tip of the spark plug on a piece of unpainted metal such as a nut, bolt or bracket to get a good ground. Next, turn the motor over to see if there’s a spark. 

Outboard engine spark plug test

Outboard motor spark plug test

If the spark is yellow or orange, assuming you got a bright light when you tested your current earlier, that means the spark plug is weak and you should change it. 

When replacing spark plugs, don’t cross-thread new plugs as you’re installing them. Also, check with the manufacturer to see if they recommend applying a thin coat of marine grade dielectric grease or non-seize lubricant on the tip of the spark plug.

Still having trouble with the ignition system of your outboard after testing out the spark plugs? Watch the video above to see more troubleshooting tips for outboard ignition system problems.