The spark plug can be read for ignition timing and for the correct heat range. But the spark plugs can only be read correctly if the engine has been shut down immediately at the end of a run. Also, with new spark plugs, it may take a couple of runs before the spark plug color is noticeable.
Reading for Ignition Timing
We've already touched on read the spark plug for ignition timing in our section on ignition timing, but to recap: you can read the ignition timing by the blue coloring on the spark plug's ground strap. This blue coloring indicates the point at which the ground strap has reached the annealing temperature of the metal. You are looking specifically for this coloring to be centered at the sharp bend or elbow of the earth strap. If it closer to the tip, your ignition timing is too far advance at that rev range for that run and for the octane level of the fuel. If it occurs closer to the threaded body of the spark plug or there is no coloration, then your timing curve needs to be adjusted to advance the ignition timing.
Reading the Heat Range
Obviously, if preignition is occurring, you will not be able to read the spark plug for ignition timing. Signs of preignition will show up near the base of the white porcelain insulator of the spark plug where you would find tiny black or brown specks or shiny specks of aluminum that have fused to the porcelain. You may need a magnifying glass to see these specks. This is usually by a spark plug taht is too hot. Using a spark plug that is one or two heat ranges lower should resolve the problem.
The ground strap is the main indicator of the spark plug's heat range. The ground strap should have light brownish deposits on the end, starting halfway between the end of the ground strap (above the central electrode) and the bend. The rest of the strap should have grayish deposits. If the brownish deposists start closer to the end of the strap, then the spark plug's heat range is a bit too cold. If the brownish deposists start closer to the bend or below the bend of the ground strap, then the spark plug's heat range is a bit too hot.
If the deposits are soft sooty black deposits, then the spark plug's heat range is much too cold or your air/fuel mixture is too rich.
The porcelain insulator should also be chalky or dull. It should not have a shiny glazed appearance. The porcelain insulator becomes glazed when the idling air/fuel mixture is too rich resulting in deposits on the porcelain insulator. These deposits will burn and create excessive porcelain temperatures causing the glass to melt. This glazing is a conductor of electricity and will eventually result on the spark jumping from the electrode to the porcelain, producing a very weak spark.
If the elctordes are covered by slightly moist sooty black deposits, then you have oil leaking into the combustion chamber. This can be caused to faulty oil rings on the pistons or failing vlave stem seals.