How to Read a Spark Plug

How to read a spark plug

Knowing how to read the spark plug colors can be a valuable aid when giving the engine a tune up. Let's look at how It is done.

The spark plug colors can let you know if the engine is firing properly or if you are beginning to run into trouble. There are five basic spark plug colors and condition combinations to judge by.

spark plug reading

Grey or slightly discolored spark plug colors are a good indicator that the engine is running properly. When the air and fuel ratio is just right, the spark plugs burn properly and there isn't a lot of surface build up.

Sooty black, dry spark plug colors and surface conditions would indicate the engine could have one or more problems. The carburetor may be set too rich, so the plugs don't burn properly, leaving behind the build up. The flame arrestor may also be too dirty to work well. Sooty spark plug colors may also indicate a problem with the ignition system causing a weak spark.

Black spark plug colors that look wet in appearance are an indication of fouling. The oil ratio, and possibly the gas ratio, is too high for a proper mix which keeps the plug from burning the way it should. It is also possible to get this kind of build up when you only drive in the city. Take the ATV out to reach higher RPMs, allowing the plugs to burn hotter and burn off the build up.

Chalky, lumpy build up spark plug colors may indicate bad fuel or an oil problem. Always be sure you are using the right grade of oil according to the owners manual. If you can rule this problem out, you might want to look for an oil leak in the combustion chamber.

If the spark plug colors look white, with bubbles or blistering, the plugs are overheating. This could be caused by a gapping or torque problem, as well as an air and fuel mix set too lean. If you can rule these problems out, check the timing, since too fast will cause the spark plugs to burn too hot.

By knowing how to read the spark plug colors, you can catch problems developing early before they do more serious damage to the engine.

Read the spark plug regularly; at least once every 2 months.

The absolute best way to to get an optimal and correct spark plug reading is by doing the following. Drive the ATV up to its last gear, full throttle and hit the clutch lever. Hold off giving more gas and turn off the engine. Then, take out the spark plug and check its color. This is the most optimal way to read what fuel/air mixture the bike is getting.

Check the spark plug by its color.

  • Fuel/air mixture is too rich: grey, black dry, black wet, chalky, white
  • Fuel/air mixture optimal: coffee with milk

If the spark plug shows a 'coffee with milk' color, it means the fuel/air mixture the bike is getting is correct and the plug does not need replacing. Other then that color, the fuel-air mixture is incorrect and needs adjusting. Before you replace the sprak plug with a new one, find and fix the problem. Read just the jetting to assure you will not have to replace the spark plug again after a few week.

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