What Does It Mean When The Spark Plug Is Black?

One way to find out if your spark plug is fouled is when its electrodes are black.

Blackened electrodes can mean one or two things: carbon fouling and oil fouling.

Blackened Spark Plugs

Spark plugs can become blackened for several reasons. Depending on how they appear, it will give you a clue about what caused it and what you should do about it. 

That is why it is critical to learn how to read the condition of your spark plugs. A fouled spark plug with blackened electrodes can cause one or more of the following problems: 

  • Engine misfires
  • Engine stalls
  • Reduced gas mileage
  • Slow acceleration
  • Hard starts

Causes Of Spark Plug Fouling | Maintenance Minute

Carbon Fouling

A carbon-fouled plug can mean you have a rich air-fuel mixture in your cylinder.

Too much rich air-fuel mixtures can leave the plug tip with excess carbon deposit, and that excess carbon accumulates on your spark plug. 

Symptoms: Dry soot on the electrodes and insulator tip; gray to matte black.

Remedy: Try consulting a mechanic for the required replacement for the damaged plug.

Oil Fouling

An oil-fouled plug indicates that oil is leaking into the cylinders. The leak is usually due to worn pistons or valve seals that allow oil to pass through.

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Symptoms: Wet texture on the electrodes and insulator tip; oily and black.

Remedy: Replace the damaged plug and seek help from a mechanic to find the source of the leak.

Frequently Asked Questions

What can I do to prevent my spark plugs from turning black?

Here are some potential solutions to this issue:

  1. Clean your spark plug – You should clean your spark plug frequently. Try doing this whenever you’re changing your engine’s oil.
  1. Fix air/fuel mixture – Set your mixture correctly. Your recommended ratio should be 14.7: 1.
  1. Narrow your spark plug gap – You have to reduce the gap of your ground electrode. Use a plug gap tool with this.
  1. Clean your air filter – A clean air filter can help prevent spark plug damage. Your recommended cleaning material should always be clean water.

How do I fix carbon-fouled spark plugs?

Although it’s wiser to replace fouled spark plugs, it’s possible to fix this problem. You can:

  1. Sandblast – You can scrape carbons off the plugs using a machine that shoots sand out with a jet of air.
  1. Use abrasives – You can also scrape off the carbons from your plug with abrasives such as sandpaper or a wire brush.
  1. Use treatment products – Products are available on the market to clean your carbon-fouled plug.
  1. Using a butane torch – Burning off carbon is another effective method for repairing your plug.

How to “Read” your Spark Plugs | Goss’ Garage

Why do black carbons accumulate on my bike’s spark plug so frequently?

  1. One reason could be that it’s a clear indication that your bike isn’t properly tuned. In other words, your bike isn’t completely burning the fuel/air mixture. 
  1. Your spark plugs may be old. If so, you should consider replacing them with new ones.
  1. You have contaminated oil. If your oil is dark and thick, it could be contaminated with dirt. Take this as a sign that your engine’s oil needs to be changed.
  1. Another possibility is that you have some internal leaks. These leaks could include oil, fuel, and air leaks.