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The condition of the spark plug tells a lot about how an engine is performing. Reading spark plugs from carbureted 2 stroke engines is more difficult than fuel injected automobile engines because the fuel mixture in carbureted engines is, basically, all over the place.
Engines that use leaded fuel of any kind will have more deposits on the spark plug which can interfere somewhat with reading the plug. However, if the FAA approved fuel additives are used with AVGAS, deposits will be almost non-existent. (See the Fuel & Oil Specifications page for more info on this subject.)
In addition to the photos below, Champion has this excellent website page with photos and explanation on reading spark plugs. The page, however, is mostly about the extremes of what happens to plugs.
Here is a healthy spark plug from an engine running non-leaded gasoline. Engine maintenance is less and pollution is lower with non-leaded fuels and these are the only advantages. A plug that is too cold will be covered with black (unburned fuel) deposits and the engine may be hard to start and not reach peak horsepower. Most paramotor manufacturers specify spark plugs for hot and cold running environments and it is important to have the correct one. Generally, if the air temperature is usually over 80F use a cooler running spark plug.
Below is a healthy spark plug from an engine running AVGAS treated with the TCP additive. The plug has about 25 hours on it and is at the end of its life. It has a trace of lead monoxide deposits which is normal. Even a plug exposed to lead-free fuel would have had some deposits after 25 hours.
This spark plug is from an AVGAS burning engine that did not have an anti-lead fouling additive mixed in. Note that lead monoxide is present close to the grounded electrode and absent from the cooler parts of the plug. The cylinder head had no lead deposits and the piston had minimal deposits. The only symptom the engine had was hard-starting because the plug is worn out. Engines that use ethanol blends will not last as long and why any other kind of fuel is preferable. Regardless of what kind of fuel is used, the spark plug will wear out and/or foul. It is so inexpensive to change out the spark plug and improve starting and high end performance and is why it should always be done.
Pilots should not change their spark plug type from the manufacturer's specification unless there is a clear and understandable reason to do so e.g. the plug is getting too hot (need a cooler plug) or fouls easily (needs a hotter plug). The quality of the fuel and the general environment where the engine is run (hot & humid or cold & dry) can help determine whether the plug should be hotter or colder.