Model A Ford Club of America
Sacramento, California Chapter

What your spark plugs can tell you

What your Sparks are Telling You. The only real way that you know what’s happening inside your engine is to read your spark plugs. Both the fuel mixture and the ignition timing result in coloring of the spark plugs porcelain and ground strap. The trick is how to get the correct coloring without going into detonation and destroying the engine or by going too rich and raising the ring lands.

Source: 
Model A Times - Fall 2013