Spark plug gap incorrect from factory?

[stickerbush]

2023 Boston Whaler, 40 HP Mercury 4 Stroke EFI.

I purchased from original owner this spring, motor had 40 hrs at that time. I removed the spark plugs today for inspection and they were all gapped to .026. The owners manual says gap should be .040. Boat was previously serviced by a reputable dealer and I seriously doubt that they did much more than change the oil. The plugs are pretty black, indicating to me that the motor is running on a bit rich. Could this be due to the tighter plug gap? I've since put 45 hrs on the motor, probably about 20% at idle speed.

 

[Miles2000]

I'd be surprised if the gap is what's making them black...

A tight gap does not add fuel. On an EFI motor the ECM sets the injector pulse width off the MAP sensor and the temp sensor. The plug gap is downstream of all that. If anything a tighter gap fires easier, it takes less voltage to jump. It's the other direction that bites you. Too wide a gap raises the voltage the coil has to make and you get a miss up on plane...

What I'd want to know first is what plug is actually in there. Read the number stamped on the metal body and hold it against what your manual calls for. SWAG: somebody grabbed a plug off the shelf that crosses to something else and never put a gauge on it. Wrong heat range will soot up no matter where you set the gap...

One caution though. If those are fine wire plugs, the iridium or platinum ones with the skinny center electrode, don't try to pry the ground strap out to .040. You'll break the electrode. Just buy the right ones, they aren't expensive...

And FWIW, reading plugs on a motor that idles a lot is close to useless. Carbon doesn't burn off until the plug gets hot, and it doesn't get hot at idle. 20% of your hours at idle will make any plug look rich. If you want a real read you have to run it hard for a while, shut it down right there, and pull them without idling back to the ramp...

45 hours at .026 didn't hurt anything. I wouldn't lose sleep over that part.

A few things that would help.

What's the number stamped on the plug?
Are they all the same color, or is one worse than the rest?
Dry and sooty, or wet and oily?
Does the temp gauge come up where it should, and is the tell-tale strong?
Does it idle smooth or is there a stumble?

If one hole is much darker than the others that points at that one injector. If they're all dark and it never really warms up, I'd look at the thermostat before I touched anything on the fuel side...

Thanks

 

[Texasmark]

Used to be that new spark plugs had all kinds of gaps. In changing plugs, you remove the plug from the carton, set the gap to your requirements and install it. On a narrower gap, the voltage doesn't get as high as it would otherwise..... .026 is a long way from .040. The wider gap came along with Capacitor Discharge Ignition's in the newer engines. Higher voltage had more "energy" in the spark and provided for more complete combustion which resulted in better engine performance. Mercury marine boasts 40,000 volts of available spark voltage to fire its plugs in its CDI systems....coil and points systems of the old days, only put out 18,000......both are unloaded voltages....spark gap setting determines where in that range the plug will fire.

On the question as to whether or not the gap led to the condition of the plugs when you removed them.......you bet your bippy! The black you see is unburned fuel components that were sent into the cylinder to explode but didn't. If your fuel isn't exploding, its not providing power to run your engine.

 

[boscoe]

While having higher voltages, capacitor discharge ignition systems don't necessarily have greater energy.

Energy is a function of voltage and current. CDI systems have higher voltages but less current flow. The spark duration in a CDI ignition system is not a long as it is in a battery powered ignition system.

I suspect the OP's model uses a battery powered ignition systems. Less voltage but longer spark duration.

 

[stickerbush]

What's the number stamped on the plug?
Champion RA8HC, same as specified in the owner's manual

Are they all the same color, or is one worse than the rest?
They are all the same.

Dry and sooty, or wet and oily?
The top one was dry, the other two were a bit wet with fuel.

Does the temp gauge come up where it should, and is the tell-tale strong?
The only temp gauge that I can see is on the Mercury phone app, it runs around 120F. The tell-tale is strong.

Does it idle smooth or is there a stumble?
The idle is generally smooth, but there are occasional hiccups until it warms up thoroughly. The motor runs great, starts easily and I've never had a problem with it.

When I queried Gemini AI it said that the larger gap results in a stronger spark that improves combustion at idle.

 

[stickerbush]