A bright blue spark is best. A yellow/orange spark signifies weak ignition. The hottest spark is ultraviolet which we can't see.
A good spark will be blue-white and will be plainly visible in daylight. If a good spark is present, the problem is probably not in the ignition system. Check the fuel system and/or stark timing. Weak sparks are orange or red and may be hard to see in daylight.
Bad Sparks
While a brief, odorless spark shouldn't worry you, it's time to contact an electrician if you see a spark that has one or more of these characteristics: It lingers and fizzles, as opposed to briefly flashing. It's noticeably white or yellow, as opposed to blue.
If the engine is running to rich the spark plug will look black, sometimes a glossy black if it's really rich. The goal is to get a good dark coffee brown color on the spark plug, which represents the correct color on the spark plug itself, and assures you that the Fuel to Air mixture on your carb is set correctly.
Make sure the spark is strong and blue in color. If the spark is weak and dull orange, it's likely that it's not strong enough to ignite the engine's air and fuel mixture. (Optional) if There Is No Spark, Connect a New or Known Working Plug Connect a good plug to the coil pack or spark plug wire and repeat the test.
The strength of the spark is revealed in the color. A red or yellow spark is weak and probably will not spark in the cylinder. A blue or white spark is strong and has enough voltage to fight across the spark plug gap even under pressure within the cylinder.
The most common signs of bad spark plugs include start-up trouble, rough idling, sluggish acceleration, declining fuel economy, engine misfiring, and engine knocking.
Black or dark brown: The engine probably runs too rich. These spark plugs will feel a bit dingy under your hands. Oily and black: Fouled oils.
Good running conditions: If everything is good, the spark plug should have a tan/light brown color. Rich running conditions: If your engine is running too rich, the spark plug will be black and sooty. Lean running conditions: If your engine is running too lean, the spark plug will be white.
Over time, the spark plugs in a healthy engine develop a slight brown or tan tint of color on the center and ground electrodes. The electrode wear is uniform and like the threads, should appear free of oil and unburned carbon.
Dark red-orange sparks are known from charcoal, iron powder leads to yellow/golden sparks, and hot burning elemental metal powders such as aluminum and titanium can form bright white sparks. The color of sparks is dominated by black body radiation.
What causes the blue spark that you see is the split second where the electrical plug and socket are close enough together that the electrical current literally jumps across the gap. This is what causes the spark and it actually happens every time you plug something in. It is completely safe and normal.
One of the key factors that makes a spark plug work well, or not, is the size of the gap between the electrodes. If the gap is too small, the spark will likely be too weak and cause the engine to run poorly or with poor efficiency.
If your car is running rich you should have a strong smell of gasoline from your car. Have you considered using an air/fuel ratio gauge? Personally, I prefer to see the exact number. I have the air/fuel ratio gauge wired into the back of my glovebox and keep the gauge inside.
A lean running condition may manifest in driveability symptoms that have not (or not yet) triggered a DTC. Customer complaints may include symptoms such as decreased fuel economy, lack of power, hesitation on acceleration, backfiring through the intake, overheating, weird smells or other problems.
So, yes, if you lean out your engine, going from 12:1 to around 14:1, your engine will run a bit hotter. But 14:1 is still "rich" by stoichiometric standards. If you lean out past 14.7:1, all the way to something like 17:1, your engine will run cooler again.
A spark plug that is running too lean will have a light tan or grayish color. This is caused by an insufficient amount of fuel in the combustion chamber which can lead to reduced power and poor performance.
Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn't have enough gasoline can cause a backfire, too. A "lean" mixture is one that doesn't have enough fuel, and too much air.
A black, feathery carbon deposit on your spark plugs can be an indication of a weak spark or an overly rich fuel mixture. Causes may include a stuck choke, misadjusted or heavy carburetor float, a leaky injector or carburetor needle valve, low coil output or high resistance in your spark plug wires.
If it the heat range is too hot, your engine could experience detonation, pre-ignition, or power loss. Most spark plug manufacturers recommend that the tip temperature remain between 500-850 degrees Celsius. Heat ranges are designated by each spark plug manufacturer with a number.
Tighten the spark plug finger-tight until the gasket reaches the cylinder head, then tighten about ½ – ⅔ turn more with a spark plug wrench. (Taper seat: About 1/16 turn more.)
Ordinary copper and nickel spark plugs should be changed every 30,000 to 50,000 miles, unless otherwise suggested by the spark plug manufacturer. Platinum and iridium spark plugs tend to last longer than standard copper and nickel spark plugs. Expect to change them every 60,000 to 150,000 miles unless otherwise noted.