If the engine is running to lean, the spark plug can look greyish white. If the engine is running to rich the spark plug will look black, sometimes a glossy black if it's really rich.
If your sparks plugs have gone out, the OBD scanner should read the codes P0300 or P0171 which means lean running engine coupled with misfires. A good way to check spark plugs at home is by doing a thorough visual inspection.
So a small intense spark is much better than a big, weak spark, especially when dealing with richer fuel mixtures and more volumes of fuel being burned. A good starting point and pretty much the all around best gap is . 035".
If the firing end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning optimally.
For most racing applications, you usually want the plug gap to be between 0.020 and 0.040 of an inch. Most engine builders seem to settle around 0.035 of an inch. Factors such as the type of ignition you run, cylinder heads, fuel and even timing can affect how much gap will work best for you.
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.
The larger the gap, the more voltage is needed to jump the gap. Most experienced tuners know that increasing the gap size increases the spark area exposed to the air-fuel mixture, which maximizes burn efficiency.
A P0171 code is set by your vehicle's powertrain control module (PCM) when it detects that the air-to-fuel ratio of your vehicle is too lean. This means that the PCM is relaying that the engine is receiving too much air into the combustion chamber and not enough fuel.
If the pilot circuit is rich, the engine will have a rough idle or may not return to idle without "blipping" the throttle. You may also notice a distinct smell of raw fuel and some eye irritation from the exhaust.
Adding in extra fuel prevents detonation and just generally makes the engine happier. 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.
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.
Engine Running Rich Symptoms
If your car is running rich, you'll likely experience these symptoms: Low gas mileage. A strong smell of gasoline, especially when idling. Check engine light warning.
A Closer Look At Spark Plugs
A yellowish color is typically caused by leaded gas. Red is a sign of iron particle s or rust in the fuel system. Black is the color of oil fouling. Pay more attention to the size of the deposits and where they occur to determine if there is a problem.
Lean Air/Fuel Mixture
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 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.
A wet spark plug likely means that it hasn't been firing due to engine flooding or a bad ignition cable. Dirt or moisture on the outside of the spark plug that provides a conductive path to ground, or an internal crack in the spark plug's ceramic insulator that shorts the plug to ground also can be the culprit.
Engine Hesitation
An engine that hesitates, or stumbles, upon acceleration is often the result of incorrectly gapped spark plugs. If spark plugs are gapped too wide, the ignition spark loses some of its strength as it crosses the spark plug gaps. This can result in engine hesitation.